linux-kernel - Re: [PATCH AUTOSEL 6.14 317/642] x86/bugs: KVM: Add support for SRSO_MSR

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Message-ID: <aCyN8IoJXk5G3eR6@lappy>
Date: Tue, 20 May 2025 10:13:04 -0400
From: Sasha Levin <sashal@...nel.org>
To: Sean Christopherson <seanjc@...gle.com>
Cc: linux-kernel@...r.kernel.org, stable@...r.kernel.org,
	Borislav Petkov <bp@...en8.de>, tglx@...utronix.de,
	peterz@...radead.org, jpoimboe@...nel.org, corbet@....net,
	mingo@...hat.com, dave.hansen@...ux.intel.com, x86@...nel.org,
	pbonzini@...hat.com, thomas.lendacky@....com,
	mario.limonciello@....com, perry.yuan@....com, kai.huang@...el.com,
	xiaoyao.li@...el.com, tony.luck@...el.com, xin3.li@...el.com,
	kan.liang@...ux.intel.com, linux-doc@...r.kernel.org,
	kvm@...r.kernel.org
Subject: Re: [PATCH AUTOSEL 6.14 317/642] x86/bugs: KVM: Add support for
 SRSO_MSR_FIX

On Mon, May 05, 2025 at 03:37:17PM -0700, Sean Christopherson wrote:
>On Mon, May 05, 2025, Sasha Levin wrote:
>> From: Borislav Petkov <bp@...en8.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@...gle.com>
>> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
>> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
>> Link: https://lore.kernel.org/r/20241202120416.6054-1-bp@kernel.org
>> Signed-off-by: Sasha Levin <sashal@...nel.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.

Sure, I'll drop it. Thanks!

-- 
Thanks,
Sasha