Re: [PATCH 0/4] KVM: x86/mmu: MMIO caching bug fixes

Michael Roth
2022-07-29T01:12:44Z

On Thu, Jul 28, 2022 at 10:17:55PM +0000, Sean Christopherson wrote:
> Fix two bugs I introduced when adding the enable_mmio_caching module param.
>
> Bug #1 is that KVM unintentionally makes disabling caching due to a config
> incompatibility "sticky", e.g. disabling caching because there are no
> reserved PA bits prevents KVM from enabling when "switching" to an EPT
> config (doesn't rely on PA bits) or when SVM adjusts the MMIO masks to
> account for C-bit shenanigans (even if MAXPHYADDR=52 and C-bit=51, there
> can be reserved PA bits due to the "real" MAXPHYADDR being reduced).
>
> Bug #2 is that KVM doesn't explicitly check that MMIO caching is enabled
> when doing SEV-ES setup. Prior to the module param, MMIO caching was
> guaranteed when SEV-ES could be enabled as SEV-ES-capable CPUs effectively
> guarantee there will be at least one reserved PA bit (see above). With
> the module param, userspace can explicitly disable MMIO caching, thus
> silently breaking SEV-ES.
>
> I believe I tested all combinations of things getting disabled and enabled
> by hacking kvm_mmu_reset_all_pte_masks() to disable MMIO caching on a much
> lower MAXPHYADDR, e.g. 43 instead of 52. That said, definitely wait for a
> thumbs up from the AMD folks before queueing.

I tested the below systems/configurations and everything looks good
to me. Thanks for the quick fix!

AMD Milan, MAXPHYADDR = 48 bits, kvm.mmio_caching=on (on by default)
normal: pass
SEV: pass
SEV-ES: pass

AMD Milan, MAXPHYADDR = 48 bits, kvm.mmio_caching=off
normal: pass
SEV: pass
SEV-ES: fail (as expected, since kvm_amd.sev_es gets forced to off)

AMD unreleased, MAXPHYADDR = 52 bits, kvm.mmio_caching=on (on by default)
normal: pass
SEV: pass
SEV-ES: pass

AMD unreleased, MAXPHYADDR = 52 bits, kvm.mmio_caching=off
normal: pass
SEV: pass
SEV-ES: fail (as expected, since kvm_amd.sev_es gets forced to off)

>
> Sean Christopherson (4):
> KVM: x86: Tag kvm_mmu_x86_module_init() with __init
> KVM: x86/mmu: Fully re-evaluate MMIO caching when SPTE masks change
> KVM: SVM: Adjust MMIO masks (for caching) before doing SEV(-ES) setup
> KVM: SVM: Disable SEV-ES support if MMIO caching is disable

Series:

Tested-by: Michael Roth <michael.roth@amd.com>

-Mike