RE: [PATCH v3 00/10] Add Intel VT-d nested translation

From: Tian, Kevin
Date: Wed May 24 2023 - 05:04:16 EST

> From: Liu, Yi L <yi.l.liu@xxxxxxxxx>
> Sent: Thursday, May 11, 2023 10:51 PM
> The first Intel platform supporting nested translation is Sapphire
> Rapids which, unfortunately, has a hardware errata [2] requiring special
> treatment. This errata happens when a stage-1 page table page (either
> level) is located in a stage-2 read-only region. In that case the IOMMU
> hardware may ignore the stage-2 RO permission and still set the A/D bit
> in stage-1 page table entries during page table walking.
> A flag IOMMU_HW_INFO_VTD_ERRATA_772415_SPR17 is introduced to
> report
> this errata to userspace. With that restriction the user should either
> disable nested translation to favor RO stage-2 mappings or ensure no
> RO stage-2 mapping to enable nested translation.
> Intel-iommu driver is armed with necessary checks to prevent such mix
> in patch10 of this series.
> Qemu currently does add RO mappings though. The vfio agent in Qemu
> simply maps all valid regions in the GPA address space which certainly
> includes RO regions e.g. vbios.
> In reality we don't know a usage relying on DMA reads from the BIOS
> region. Hence finding a way to allow user opt-out RO mappings in
> Qemu might be an acceptable tradeoff. But how to achieve it cleanly
> needs more discussion in Qemu community. For now we just hacked Qemu
> to test.

Hi, Alex,

Want to touch base on your thoughts about this errata before we
actually go to discuss how to handle it in Qemu.

Overall it affects all Sapphire Rapids platforms. Fully disabling nested
translation in the kernel just for this rare vulnerability sounds an overkill.

So we decide to enforce the exclusive check (RO in stage-2 vs. nesting)
in the kernel and expose the restriction to userspace so the VMM can
choose which one to enable based on its own requirement.

At least this looks a reasonable tradeoff to some proprietary VMMs
which never adds RO mappings in stage-2 today.

But we do want to get Qemu support nested translation on those
platform as the widely-used reference VMM!

Do you see any major oversight before pursuing such change in Qemu
e.g. having a way for the user to opt-out adding RO mappings in stage-2? 😊