linux-kernel - Re: [PATCH 1/3] x86/bugs: Use VM_CLEAR_CPU

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Message-ID: <DDWGY8JOYRIO.2XYJMYGEEVLIX@google.com>
Date: Fri, 31 Oct 2025 11:25:49 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, 
	Brendan Jackman <jackmanb@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>, 
	Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>, 
	Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>, 
	"H. Peter Anvin" <hpa@...or.com>, Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>, 
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>, 
	Tao Zhang <tao1.zhang@...el.com>, Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 1/3] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

On Thu Oct 30, 2025 at 6:43 PM UTC, Pawan Gupta wrote:
> On Thu, Oct 30, 2025 at 12:28:06PM +0000, Brendan Jackman wrote:
>> On Wed Oct 29, 2025 at 9:26 PM UTC, Pawan Gupta wrote:
>> > TSA mitigation:
>> >
>> >   d8010d4ba43e ("x86/bugs: Add a Transient Scheduler Attacks mitigation")
>> >
>> > introduced VM_CLEAR_CPU_BUFFERS for guests on AMD CPUs. Currently on Intel
>> > CLEAR_CPU_BUFFERS is being used for guests which has a much broader scope
>> > (kernel->user also).
>> >
>> > Make mitigations on Intel consistent with TSA. This would help handling the
>> > guest-only mitigations better in future.
>> >
>> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
>> > ---
>> >  arch/x86/kernel/cpu/bugs.c | 9 +++++++--
>> >  arch/x86/kvm/vmx/vmenter.S | 3 ++-
>> >  2 files changed, 9 insertions(+), 3 deletions(-)
>> >
>> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> > index d7fa03bf51b4517c12cc68e7c441f7589a4983d1..6d00a9ea7b4f28da291114a7a096b26cc129b57e 100644
>> > --- a/arch/x86/kernel/cpu/bugs.c
>> > +++ b/arch/x86/kernel/cpu/bugs.c
>> > @@ -194,7 +194,7 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
>> >  
>> >  /*
>> >   * Controls CPU Fill buffer clear before VMenter. This is a subset of
>> > - * X86_FEATURE_CLEAR_CPU_BUF, and should only be enabled when KVM-only
>> > + * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
>> >   * mitigation is required.
>> >   */
>> 
>> So if I understand correctly with this patch the aim is:
>> 
>> X86_FEATURE_CLEAR_CPU_BUF means verw before exit to usermode
>> 
>> X86_FEATURE_CLEAR_CPU_BUF_VM means unconditional verw before VM Enter
>> 
>> cpu_buf_vm_clear[_mmio_only] means verw before VM Enter for
>> MMIO-capable guests.
>
> Yup, thats the goal.
>
>> Since this is being cleaned up can we also:
>> 
>> - Update the definition of X86_FEATURE_CLEAR_CPU_BUF in cpufeatures.h to
>>   say what context it applies to (now it's specifically exit to user)
>> 
>> - Clear up how verw_clear_cpu_buf_mitigation_selected relates to these
>>   two flags. Thinking aloud here... it looks like this is set:
>> 
>>   - If MDS mitigations are on, meaning both flags are set
>> 
>>   - If TAA mitigations are on, meaning both flags are set
>> 
>>   - If MMIO mitigations are on, and the CPU has MDS or TAA. In this case
>>     both flags are set, but this causality is messier.
>> 
>>   - If RFDS mitigations are on and supported, meaning both flags are set
>> 
>>   So if I'm reading this correctly whenever
>>   verw_clear_cpu_buf_mitigation_selected we should expect both flags
>>   enabled. So I think all that's needed is to add a reference to
>>   X86_FEATURE_CLEAR_CPU_BUF_VM to the comment?
>
> Yes. I will update the comment accordingly.
>
>> I think we also need to update the assertion of vmx->disable_fb_clear?
>
> I am not quite sure about the update needed. Could you please clarify?
>
>> Anyway thanks this seems like a very clear improvement to me.
>
> Thanks for the review and suggestions!

I will drop this thread and continue here:
https://lore.kernel.org/all/20251031003040.3491385-2-seanjc@google.com/