Messages in this thread | | | Date | Thu, 16 Feb 2023 08:42:53 +0800 | Subject | Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment | From | Baolu Lu <> |
| |
On 2/15/23 7:09 PM, Robin Murphy wrote: > On 2023-02-15 07:28, Baolu Lu wrote: >> On 2023/2/15 14:56, Tian, Kevin wrote: >>>> From: Baolu Lu<baolu.lu@linux.intel.com> >>>> Sent: Wednesday, February 15, 2023 1:51 PM >>>> >>>> On 2/13/23 10:19 PM, Jason Gunthorpe wrote: >>>>> On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote: >>>>>> @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct >>>> iommu_group *group, >>>>>> else >>>>>> return -EINVAL; >>>>>> >>>>>> + if (req_type != IOMMU_DOMAIN_DMA_FQ || >>>>>> + group->default_domain->type != IOMMU_DOMAIN_DMA) { >>>>>> + ret = iommu_group_claim_dma_owner(group, (void *)buf); >>>>>> + if (ret) >>>>>> + return ret; >>>>>> + group_owner_claimed = true; >>>>>> + } >>>>> I don't get it, this should be done unconditionally. If we couldn't >>>>> take ownership then we simply can't progress. >>>> The existing code allows the user to switch the default domain from >>>> strict to lazy invalidation mode. The default domain is not changed, >>>> hence it should be seamless and transparent to the device driver. >>> Is there real usage relying on this transition for a bound device? >>> >>> In concept strict->lazy transition implies relaxed DMA security. It's >>> hard >>> to think of a motivation of doing so while the device might be doing >>> in-fly DMAs. >>> >>> Presumably such perf/security tradeoff should be planned way before >>> binding device/driver together. >>> >>> btw if strict->lazy is allowed why lazy->strict is prohibited? >>> >> >> We all know, strict vs. lazy is a tradeoff between performance and >> security. >> >> strict -> lazy: driver works in secure mode. This transition trades off >> security for better performance. >> >> lazy->strict: The driver is already working in non-safety mode. This >> transition only results in worse performance. It makes no sense. If user >> want to put the driver in a secure mode, they need to unbind the driver, >> reset the device and do the lazy->strict transition. >> >> Robin might have better insights. > > Yes, this was added for a definite use-case in ChromeOS, where > strict->lazy needs to support being done "live" since the device in > question is the storage controller for the already-mounted root > filesystem.
Thanks for letting us know this.
> Your reasoning seems to match what I summarised in the > original commit message 😄
Haha, it seems that my memory is till good. :-)
Best regards, baolu
| |