Re: [PATCH AUTOSEL 6.14 317/642] x86/bugs: KVM: Add support for SRSO_MSR_FIX

[Sean Christopherson <seanjc@google.com>]

On Mon, May 05, 2025, Sasha Levin wrote:
> From: Borislav Petkov <bp@alien8.de>
> 
> [ 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: Sasha Levin <sashal@kernel.org>
> ---

Can we please hold off on this until the fix lands[1]?  This version introduces
a very measurable performance regression[2] for non-KVM use cases.

[1] https://lore.kernel.org/all/20250502223456.887618-1-seanjc@google.com
[2] https://www.phoronix.com/review/linux-615-amd-regression