linux-kernel - Re: Possible 5.19 regression for systems with 52-bit physical address support

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Date:   Thu, 28 Jul 2022 15:43:07 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Tom Lendacky <thomas.lendacky@....com>
Subject: Re: Possible 5.19 regression for systems with 52-bit physical
 address support

On Thu, Jul 28, 2022, Sean Christopherson wrote:
> On Thu, Jul 28, 2022, Michael Roth wrote:
> > On Thu, Jul 28, 2022 at 08:44:30AM -0500, Michael Roth wrote:
> Different approach.  To fix the bug with enable_mmio_caching not being set back to
> true when a vendor-specific mask allows caching, I believe the below will do the
> trick.

...
 
> diff --git a/arch/x86/kvm/mmu/spte.c b/arch/x86/kvm/mmu/spte.c
> index 7314d27d57a4..a57add994b8d 100644
> --- a/arch/x86/kvm/mmu/spte.c
> +++ b/arch/x86/kvm/mmu/spte.c
> @@ -19,8 +19,9 @@
>  #include <asm/memtype.h>
>  #include <asm/vmx.h>
> 
> -bool __read_mostly enable_mmio_caching = true;
> -module_param_named(mmio_caching, enable_mmio_caching, bool, 0444);
> +bool __read_mostly enable_mmio_caching;
> +static bool __read_mostly __enable_mmio_caching = true;
> +module_param_named(mmio_caching, __enable_mmio_caching, bool, 0444);
> 
>  u64 __read_mostly shadow_host_writable_mask;
>  u64 __read_mostly shadow_mmu_writable_mask;
> @@ -340,6 +341,8 @@ void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 mmio_mask, u64 access_mask)
>         BUG_ON((u64)(unsigned)access_mask != access_mask);
>         WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
> 
> +       enable_mmio_caching = __enable_mmio_caching;

This isn't ideal as the value used by KVM won't be reflected in the module param.
The basic approach is sound, but KVM should snapshot the original value of the module
param and "reset" to that.

> +
>         if (!enable_mmio_caching)
>                 mmio_value = 0;
> 
>