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    SubjectRe: [PATCH v3 2/3] uio_pci_generic: add MSI/MSI-X support
    On Thu, Oct 08, 2015 at 04:28:34PM +0300, Gleb Natapov wrote:
    > On Thu, Oct 08, 2015 at 04:20:04PM +0300, Michael S. Tsirkin wrote:
    > > On Thu, Oct 08, 2015 at 03:27:37PM +0300, Gleb Natapov wrote:
    > > > On Thu, Oct 08, 2015 at 03:06:07PM +0300, Michael S. Tsirkin wrote:
    > > > > On Thu, Oct 08, 2015 at 12:44:09PM +0300, Avi Kivity wrote:
    > > > > >
    > > > > >
    > > > > > On 10/08/2015 12:16 PM, Michael S. Tsirkin wrote:
    > > > > > >On Thu, Oct 08, 2015 at 11:46:30AM +0300, Avi Kivity wrote:
    > > > > > >>
    > > > > > >>On 10/08/2015 10:32 AM, Michael S. Tsirkin wrote:
    > > > > > >>>On Thu, Oct 08, 2015 at 08:33:45AM +0300, Avi Kivity wrote:
    > > > > > >>>>It is good practice to defend against root oopsing the kernel, but in some
    > > > > > >>>>cases it cannot be achieved.
    > > > > > >>>Absolutely. That's one of the issues with these patches. They don't even
    > > > > > >>>try where it's absolutely possible.
    > > > > > >>>
    > > > > > >>Are you referring to blocking the maps of the msix BAR areas?
    > > > > > >For example. There are more. I listed some of the issues on the mailing
    > > > > > >list, and I might have missed some. VFIO has code to address all this,
    > > > > > >people should share code to avoid duplication, or at least read it
    > > > > > >to understand the issues.
    > > > > >
    > > > > > All but one of those are unrelated to the patch that adds msix support.
    > > > >
    > > > > They are related because msix support enables bus mastering. Without it
    > > > > device is passive and can't harm anyone. With it, suddently you need to
    > > > > be very careful with the device to avoid corrupting kernel memory.
    > > > >
    > > > Most (if not all) uio_pci_generic users enable pci bus mastering. The
    > > > fact that they do that without even tainting the kernel like the patch
    > > > does make current situation much worse that with the patch.
    > >
    > > It isn't worse. It's a sane interface. Whoever enables bus mastering
    > > must be careful. If userspace enables bus mastering then userspace
    > > needs to be very careful with the device to avoid corrupting kernel
    > > memory. If kernel does it, it's kernel's responsibility.
    > >
    > Although this definition of sanity sounds strange to me, but lets
    > flow with it for the sake of this email: would it be OK if proposed
    > interface refused to work if bus mastering is not already enabled by
    > userspace?

    An interface could be acceptable if there's a fallback where it
    works without BM but slower (e.g. poll pending bits).

    But not the proposed one.

    Really, there's more to making msi-x work with
    userspace drivers than this patch. As I keep telling people, you would
    basically reimplement vfio/pci. Go over it, and see for yourself.
    Almost everything it does is relevant for msi-x. It's just wrong to
    duplicate so much code.


    > --
    > Gleb.


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