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

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Message-ID: <20220729010924.zatnwgprojuqspna@amd.com>
Date:   Thu, 28 Jul 2022 20:09:24 -0500
From:   Michael Roth <michael.roth@....com>
To:     Sean Christopherson <seanjc@...gle.com>
CC:     Paolo Bonzini <pbonzini@...hat.com>, <kvm@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 0/4] KVM: x86/mmu: MMIO caching bug fixes

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@....com>

-Mike