Messages in this thread Patch in this message | | | Date | Tue, 14 Apr 2020 10:13:04 -0700 | From | Jacob Pan <> | Subject | Re: [PATCH v2 1/3] iommu/uapi: Define uapi version and capabilities |
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On Tue, 14 Apr 2020 10:13:58 -0600 Alex Williamson <alex.williamson@redhat.com> wrote:
> On Mon, 13 Apr 2020 22:05:15 -0700 > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote: > > > Hi Alex, > > Thanks a lot for the feedback, my comments inline. > > > > On Mon, 13 Apr 2020 16:21:29 -0600 > > Alex Williamson <alex.williamson@redhat.com> wrote: > > > > > On Mon, 13 Apr 2020 13:41:57 -0700 > > > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote: > > > > > > > Hi All, > > > > > > > > Just a gentle reminder, any feedback on the options I listed > > > > below? New ideas will be even better. > > > > > > > > Christoph, does the explanation make sense to you? We do have > > > > the capability/flag based scheme for IOMMU API extension, the > > > > version is mainly used for size lookup. Compatibility checking > > > > is another use of the version, it makes checking easy when a > > > > vIOMMU is launched. > > > > > > > > Thanks, > > > > > > > > Jacob > > > > > > > > On Thu, 2 Apr 2020 11:36:04 -0700 > > > > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote: > > > > > > > > > On Wed, 1 Apr 2020 05:32:21 +0000 > > > > > "Tian, Kevin" <kevin.tian@intel.com> wrote: > > > > > > > > > > > > From: Jacob Pan <jacob.jun.pan@linux.intel.com> > > > > > > > Sent: Tuesday, March 31, 2020 11:55 PM > > > > > > > > > > > > > > On Tue, 31 Mar 2020 06:06:38 +0000 > > > > > > > "Tian, Kevin" <kevin.tian@intel.com> wrote: > > > > > > > > > > > > > > > > From: Jacob Pan <jacob.jun.pan@linux.intel.com> > > > > > > > > > Sent: Tuesday, March 31, 2020 12:08 AM > > > > > > > > > > > > > > > > > > On Mon, 30 Mar 2020 05:40:40 +0000 > > > > > > > > > "Tian, Kevin" <kevin.tian@intel.com> wrote: > > > > > > > > > > > > > > > > > > > > From: Jacob Pan <jacob.jun.pan@linux.intel.com> > > > > > > > > > > > Sent: Saturday, March 28, 2020 7:54 AM > > > > > > > > > > > > > > > > > > > > > > On Fri, 27 Mar 2020 00:47:02 -0700 > > > > > > > > > > > Christoph Hellwig <hch@infradead.org> wrote: > > > > > > > > > > > > > > > > > > > > > > > On Fri, Mar 27, 2020 at 02:49:55AM +0000, Tian, > > > > > > > > > > > > Kevin wrote: > > > > > > > > > > > > > If those API calls are inter-dependent for > > > > > > > > > > > > > composing a feature (e.g. SVA), shouldn't we > > > > > > > > > > > > > need a way to check them together before > > > > > > > > > > > > > exposing the feature to the guest, e.g. > > > > > > > > > > > > > through a iommu_get_uapi_capabilities > > > > > > > > > > > > > interface? > > > > > > > > > > > > > > > > > > > > > > > > Yes, that makes sense. The important bit is to > > > > > > > > > > > > have a capability flags and not version > > > > > > > > > > > > numbers. > > > > > > > > > > > > > > > > > > > > > > The challenge is that there are two consumers in > > > > > > > > > > > the kernel for this. 1. VFIO only look for > > > > > > > > > > > compatibility, and size of each data struct such > > > > > > > > > > > that it can copy_from_user. > > > > > > > > > > > > > > > > > > > > > > 2. IOMMU driver, the "real consumer" of the > > > > > > > > > > > content. > > > > > > > > > > > > > > > > > > > > > > For 2, I agree and we do plan to use the > > > > > > > > > > > capability flags to check content and maintain > > > > > > > > > > > backward compatibility etc. > > > > > > > > > > > > > > > > > > > > > > For VFIO, it is difficult to do size look up > > > > > > > > > > > based on capability flags. > > > > > > > > > > > > > > > > > > > > Can you elaborate the difficulty in VFIO? if, as > > > > > > > > > > Christoph Hellwig pointed out, version number is > > > > > > > > > > already avoided everywhere, it is interesting to > > > > > > > > > > know whether this work becomes a real exception or > > > > > > > > > > just requires a different mindset. > > > > > > > > > From VFIO p.o.v. the IOMMU UAPI data is opaque, it > > > > > > > > > only needs to do two things: > > > > > > > > > 1. is the UAPI compatible? > > > > > > > > > 2. what is the size to copy? > > > > > > > > > > > > > > > > > > If you look at the version number, this is really a > > > > > > > > > "version as size" lookup, as provided by the helper > > > > > > > > > function in this patch. An example can be the newly > > > > > > > > > introduced clone3 syscall. > > > > > > > > > https://lwn.net/Articles/792628/ In clone3, new > > > > > > > > > version must have new size. The slight difference > > > > > > > > > here is that, unlike clone3, we have multiple data > > > > > > > > > structures instead of a single struct clone_args {}. > > > > > > > > > And each struct has flags to enumerate its contents > > > > > > > > > besides size. > > > > > > > > > > > > > > > > Thanks for providing that link. However clone3 doesn't > > > > > > > > include a version field to do "version as size" lookup. > > > > > > > > Instead, as you said, it includes a size parameter which > > > > > > > > sounds like the option 3 (argsz) listed below. > > > > > > > > > > > > > > > Right, there is no version in clone3. size = version. I > > > > > > > view this as a 1:1 lookup. > > > > > > > > > > > > > > > > > > > > > > > > > Besides breaching data abstraction, if VFIO has to > > > > > > > > > check IOMMU flags to determine the sizes, it has many > > > > > > > > > combinations. > > > > > > > > > > > > > > > > > > We also separate the responsibilities into two parts > > > > > > > > > 1. compatibility - version, size by VFIO > > > > > > > > > 2. sanity check - capability flags - by > > > > > > > > > IOMMU > > > > > > > > > > > > > > > > I feel argsz+flags approach can perfectly meet above > > > > > > > > requirement. The userspace set the size and flags for > > > > > > > > whatever capabilities it uses, and VFIO simply copies > > > > > > > > the parameters by size and pass to IOMMU for further > > > > > > > > sanity check. Of course the assumption is that we do > > > > > > > > provide an interface for userspace to enumerate all > > > > > > > > supported capabilities. > > > > > > > You cannot trust user for argsz. the size to be copied > > > > > > > from user must be based on knowledge in kernel. That is > > > > > > > why we have this version to size lookup. > > > > > > > > > > > > > > In VFIO, the size to copy is based on knowledge of each > > > > > > > VFIO UAPI structures and VFIO flags. But here the flags > > > > > > > are IOMMU UAPI flags. As you pointed out in another > > > > > > > thread, VFIO is one user. > > > > > > > > > > > > If that is the case, can we let VFIO only copy its own UAPI > > > > > > fields while simply passing the user pointer of IOMMU UAPI > > > > > > structure to IOMMU driver for further size check and copy? > > > > > > Otherwise we are entering a dead end that VFIO doesn't want > > > > > > to parse a structure which is not defined by him while using > > > > > > version to represent the black box size is considered as a > > > > > > discarded scheme and doesn't scale well... > > > > > I think this could be an other viable option. Let me try to > > > > > summarize since this has been a long discussion since the > > > > > original version. > > > > > > > > > > Problem statements: > > > > > 1. When launching vIOMMU in the guest, how can we ensure the > > > > > host has compatible support upfront? as compared to fail > > > > > later. > > > > > > This sounds like a feature/extension interface, both KVM and vfio > > > have them to allow userspace to check support of specific > > > features. > > Yes, the specific features are the APIs: > > - bind guest PASID > > - cache invalidation > > - page requests & response > > > > > > > 2. As UAPI data gets extended (both in size and flags), how > > > > > can we know the size to copy > > > > > > For vfio we of course use the argsz/flags trick where the user > > > tells us how big the buffer is and flags in the header tell us > > > what fields beyond the base specification are enabled. This can > > > get tricky to extend and there can be confusion whether a flag > > > indicates the presence of a field or the validity of a field. > > > > > > We also have interfaces where the ioctl is a header plus a data > > > blob where flags tell us what the data is. These can serve > > > double duty as a extension check too as we've done for > > > VFIO_DEVICE_FEATURE. This doesn't really support extension of a > > > defined feature though, rather we'd be more likely to create a > > > set of flags that indicate the data object is feature-v2 and > > > redefine the structure, or of course we revisit the entire > > > featuring question within the structure of that data blob. > > > > > > We also implement capability chains, though they're more meant for > > > passing data to the user, where the user provides a buffer and we > > > link capabilities together within that buffer for the user to > > > walk. We've defined a mechanism through -ENOSPC and argsz to > > > tell the user how large a buffer is necessary. I dare mention we > > > have a version per capability as these are largely modeled after > > > capability chains in PCI config space. We haven't actually > > > incremented any versions, but I imagine we'd do so like PCI, > > > maintaining backwards compatibility and only defining unused bits > > > and adding fields as the version increases. > > I guess capability chain is more suitable since the IOCTL uses > > container FD instead of device FD in VFIO_DEVICE_FEATURE? > > > > We can give that a try by treating IOMMU UAPIs as capabilities. That > > would address problem #1. We really need to check compatibility > > upfront in that there is no way to fail some of the UAPIs. e.g. > > unbind guest PASID, cache invalidation. > > > > > Is the objection to a global version or to any version fields? I > > > don't really understand the global version, I'd think a mechanism > > > to check extensions plus a per structure flags/version would be > > > preferred. The former should resolve how userspace can test > > > support for features requiring multiple interfaces. > > Currently we already have individual version & flags per UAPI data. > > The reason why I introduced a global/unifier is to simplify the > > compatibility checking. Global version is optional. > > With individual version & flags, user may have to keep track of > > combinations of per structure versions. > > A user would only need to meet the minimum uapi matching the feature > they want to support, right? > Yes, then no need for checking all combinations. I guess validation cases have to cover all legal combinations, but not an issue.
> > > A global version also > > > implies that we're only ever adding features and never removing. > > > For example, feature Foo is added in version 4, but it's replaced > > > by feature Bar in version 5, now userspace can't simply test > > > version >= 4 must include feature Foo. > > > > > Yes, this is why I was hoping to stick with the rule: open for > > extension, closed to modification. > > It also makes the code backward compatible easy since there old code > > would have no change when adding new features. > > But we need a way to deprecate things. The version interface is not > good for that. > Agreed, flags are good for deprecation.
> > > It seems to me that version and flags can also be complimentary, > > > for example a field might be defined by a version but a flag could > > > indicate if it's implemented. With only the flag, we'd infer the > > > field from the flag, with only the version we'd need to assume the > > > field is always implemented. So I have a hard time making a > > > blanket statement that all versions fields should be avoided. > > > > > > > > 3. Maintain backward compatibility while allowing extensions? > > > > > > > > > > I think we all agreed that using flags (capability or types) > > > > > is the way to address #3. As Christoph pointed out, version > > > > > number should not be used for this purpose. > > > > > > > > > > So for problem 1 & 2, we have the following options: > > > > > 1. Have a version-size mapping as proposed in this set. VFIO > > > > > copies from user the correct size based on version-type > > > > > lookup. Processing of the data is based on flags in IOMMU > > > > > driver. > > > > > > > > > > 2. VFIO copy its own minsz then pass the user pointer to IOMMU > > > > > driver for further copy_from_user based on flags. (by Kevin) > > > > > > > > > > 3. Adopt VFIO argsz scheme, caller fills in argsz for the > > > > > offset the variable size union. VFIO do not check argsz in > > > > > that it requires IOMMU specific knowledge. IOMMU driver Use > > > > > flags to handle the variable size.(by Alex). I think this > > > > > what we have in Yi's VFIO & QEMU patch. argsz filled by QEMU > > > > > includes bind_data. > > > > > > > > > > 4. Do not use a unified version, have a fixed size of all UAPI > > > > > structures, padding in struct and union. (Wasteful, not > > > > > preferred per V1 discussion) > > > > > > > > > > For both 2 & 3, a unified version is not used, each API > > > > > treated separately. vIOMMU will be launched w/o assurance of > > > > > compatibility of all APIs. Fault handling may be more complex > > > > > in normal operations. > > > > > > > > > > Appreciate everyone's input. Joerg and Alex, could you help to > > > > > make a decision here? > > > > > > As above, I think using a global API version number to imply > > > support for a feature is doomed to fail, we should instead expose > > > an interface to check for specific features. > > I agree. I feel we can use the capability chain at container level > > as you mentioned, right? > > We could either do a capability chain on the IOMMU_GET_INFO ioctl or > we have the CHECK_EXTENSION ioctl. The latter is quite a bit easier > to implement, but limited in what it can expose and the semantics > around checking extensions before vs after an IOMMU model is set can > be confusing. I didn't have a strong preference between them when > suggesting Kirti use one of these to indicate dirty page tracking, but > it would be nice if both features used the same mechanism > Sounds good. Just to clarify, similar to Yi's patch that uses CHECK_EXTENSION https://lkml.org/lkml/2020/3/22/109
But instead of checking for the global UAPI version, we check for each feature. i.e. diff --git a/include/uapi/linux/vfio.h b/include/uapi/linux/vfio.h index b959d0a37c4f..639e3c970b31 100644 --- a/include/uapi/linux/vfio.h +++ b/include/uapi/linux/vfio.h @@ -46,6 +46,9 @@ * code will taint the host kernel and should be used with extrem */ #define VFIO_NOIOMMU_IOMMU 8 +#define VFIO_IOMMU_BIND_GPASID 9 +#define VFIO_IOMMU_INVALIDATE 10 +#define VFIO_IOMMU_PAGE_REQUEST 11
Applications such as QEMU would include uapi/iommu.h, which has the IOMMU UAPI extensions defined. e.g. #define IOMMU_GPASID_BIND_VERSION_1
When VM starts, it will check all uAPI extensions returned by the kernel against what is in its header, e.g.
extension = ioctl(s->container, VFIO_CHECK_EXTENSION, VFIO_IOMMU_BIND_GPASID); if (extension >= IOMMU_GPASID_BIND_VERSION_1) { /* Kernel has newer extensions, we are good */ } else { stop_viommu(); } > > > In any of the proposed solutions, the > > > IOMMU driver is ultimately responsible for validating the user > > > data, so do we want vfio performing the copy_from_user() to an > > > object that could later be assumed to be sanitized, or should > > > vfio just pass a user pointer to make it obvious that the > > > consumer is responsible for all the user protections? Seems like > > > the latter. > > I like the latter as well. > > > > > That still really > > > doesn't address what's in that user data blob yet, but the vfio > > > interface could be: > > > > > > struct { > > > __u32 argsz; > > > __u32 flags; > > > __u8 data[]; > > > } > > > > > > Where flags might be partitioned like we do for DEVICE_FEATURE to > > > indicate the format of data and what vfio should do with it, and > > > data might simply be defined as a (__u64 __user *). > > > > > So, __user * will be passed to IOMMU driver if VFIO checks minsz > > include flags and they are valid. > > IOMMU driver can copy the rest based on the mandatory version/minsz > > and flags in the IOMMU uAPI structs. > > Does it sound right? This is really choice #2. > > Sounds like each IOMMU UAPI struct just needs to have an embedded size > and flags field, but yes. > Yes, an argsz field can be added to each UAPI. There are already flags or the equivalent. IOMMU driver can process the __user * based on the argsz, flags, check argsz against offsetofend(iommu_uapi_struct, last_element), etc.;
> > > This user pointer would then likely be an IOMMU UAPI struct, so > > > I've only just gotten back the the IOMMU UAPI question at hand, > > > but I don't really see the disadvantage to including both version > > > and flags fields per structure. Perhaps this is choice 1. above, > > > but with a version at a per structure level indicating the > > > backwards compatible size and layout of the structure and flags > > > being used to indicate support for optional features within those > > > fields. > > > > Per structure version & flags is what we have in the mainline. It > > applies to both choice 1 & 2. The global/unified version is just a > > re-interpretation of the versions such that we have a monolithic > > incrementing version. Again, global version is optional. > > > > > Is a version field still taboo > > > for such a use case? Thanks, > > > > I will leave that to Christoph :) > > Looks like if we drop the global version in favor of an > extension/feature check and embed size and flags fields in each IOMMU > UAPI struct, Christoph will be satisfied. Thanks, > Agreed, we will work towards it. Thanks a lot!
> Alex >
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