Re: [PATCH 6.6.y v1 5/6] x86/bugs: KVM: Add support for SRSO_MSR_FIX

[Sean Christopherson <seanjc@google.com>]

On Wed, Apr 29, 2026, Daniil Tatianin wrote:
> [ Upstream commit 8442df2b49ed9bcd67833ad4f091d15ac91efd00 ]
> 
> Add support for
> 
>   CPUID Fn8000_0021_EAX[31] (SRSO_MSR_FIX). If this bit is 1, it
>   indicates that software may use MSR BP_CFG[BpSpecReduce] to mitigate
>   SRSO.
> 
> Enable BpSpecReduce to mitigate SRSO across guest/host boundaries.
> 
> Switch back to enabling the bit when virtualization is enabled and to
> clear the bit when virtualization is disabled because using a MSR slot
> would clear the bit when the guest is exited and any training the guest
> has done, would potentially influence the host kernel when execution
> enters the kernel and hasn't VMRUN the guest yet.
> 
> More detail on the public thread in Link below.
> 
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
> Link: https://lore.kernel.org/r/20241202120416.6054-1-bp@kernel.org
> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
> ---
>  Documentation/admin-guide/hw-vuln/srso.rst | 13 ++++++++++++
>  arch/x86/include/asm/cpufeatures.h         |  4 ++++
>  arch/x86/include/asm/msr-index.h           |  1 +
>  arch/x86/kernel/cpu/bugs.c                 | 24 ++++++++++++++++++----
>  arch/x86/kvm/svm/svm.c                     |  6 ++++++
>  arch/x86/lib/msr.c                         |  2 ++
>  6 files changed, 46 insertions(+), 4 deletions(-)

For the KVM changes,

Acked-by: Sean Christopherson <seanjc@google.com>