Re: [PATCH v2] iommu/amd: Enforce 4k mapping for certain IOMMU data structures

[Brijesh Singh]

On 11/19/20 8:30 PM, Suravee Suthikulpanit wrote:
> Will,
>
> To answer your questions from v1 thread.
>
> On 11/18/20 5:57 AM, Will Deacon wrote:
> > On 11/5/20 9:58 PM, Suravee Suthikulpanit wrote:
> >> AMD IOMMU requires 4k-aligned pages for the event log, the PPR log,
> >> and the completion wait write-back regions. However, when allocating
> >> the pages, they could be part of large mapping (e.g. 2M) page.
> >> This causes #PF due to the SNP RMP hardware enforces the check based
> >> on the page level for these data structures.
> >
> > Please could you include an example backtrace here?
>
> Unfortunately, we don't actually have the backtrace available here.
> This information is based on the SEV-SNP specification.
>
> >> So, fix by calling set_memory_4k() on the allocated pages.
> >
> > I think I'm missing something here. set_memory_4k() will break the
> kernel
> > linear mapping up into page granular mappings, but the IOMMU isn't
> using
> > that mapping, right?
>
> That's correct. This does not affect the IOMMU, but it affects the PSP
> FW.
>
> > It's just using the physical address returned by
> iommu_virt_to_phys(), so why does it matter?
> >
> > Just be nice to capture some of this rationale in the log,
> especially as
> > I'm not familiar with this device.
>
> According to the AMD SEV-SNP white paper
> (https://www.amd.com/system/files/TechDocs/SEV-SNP-strengthening-vm-isolation-with-integrity-protection-and-more.pdf),
> the Reverse Map Table (RMP) contains one entry for every 4K page of
> DRAM that may be used by the VM. In this case, the pages allocated by
> the IOMMU driver are added as 4K entries in the RMP table by the
> SEV-SNP FW.
>
> During the page table walk, the RMP checks if the page is owned by the
> hypervisor. Without calling set_memory_4k() to break the mapping up
> into 4K pages, pages could end up being part of large mapping (e.g. 2M
> page), in which the page access would be denied and result in #PF.


Since the page is added as a 4K page in the RMP table by the SEV-SNP FW,
so we need to split the physmap to ensure that this page will be access
with a 4K mapping from the x86. If the page is part of large page then
write access will cause a RMP violation (i.e #PF), this is because SNP
hardware enforce that the CPU page level walk must match with page-level
programmed in the RMP table.


>
> >> Fixes: commit c69d89aff393 ("iommu/amd: Use 4K page for completion
> wait write-back semaphore")
> >
> > I couldn't figure out how that commit could cause this problem.
> Please can
> > you explain that to me?
>
> Hope this helps clarify. If so, I'll update the commit log and send
> out V3.
>
> Thanks,
> Suravee