Re: [PATCH v4 1/8] x86/bugs: Use VM_CLEAR_CPU_BUFFERS in VMX as well

[Pawan Gupta]

On Fri, Oct 31, 2025 at 11:30:54AM +0000, Brendan Jackman wrote:
> Rewording my comments from:
> https://lore.kernel.org/all/20251029-verw-vm-v1-1-babf9b961519@xxxxxxxxxxxxxxx/
> 
> On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> > From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxxxx>
> > [sean: make CLEAR_CPU_BUF_VM mutually exclusive with the MMIO mitigation]
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> 
> I think this is a clear improvement. Now that X86_FEATURE_CLEAR_CPU_BUF
> has a clear scope, can we also update the comment on its definition in
> cpufeatures.h? I.e. say that it's specifically about exit to user.

Does this suffice?

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 08ed5a2e46a5..e842f27a1108 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -321,6 +321,7 @@
 #endif
 .endm
 
+/* Primarily used in exit-to-userspace path */
 #define CLEAR_CPU_BUFFERS \
 	__CLEAR_CPU_BUFFERS X86_FEATURE_CLEAR_CPU_BUF
 

> This also seems like a good moment to update the comment on
> verw_clear_cpu_buf_mitigation_selected to mention the _VM flag too.

As we have 3 different flags, referring them with X86_FEATURE_CLEAR_CPU_*
should be okay?

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 723666a1357e..51dec95a9af5 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -490,7 +490,7 @@ static enum rfds_mitigations rfds_mitigation __ro_after_init =
 
 /*
  * Set if any of MDS/TAA/MMIO/RFDS are going to enable VERW clearing
- * through X86_FEATURE_CLEAR_CPU_BUF on kernel and guest entry.
+ * through X86_FEATURE_CLEAR_CPU_* on kernel and guest entry.
  */
 static bool verw_clear_cpu_buf_mitigation_selected __ro_after_init;
 

> Also, where we set vmx->disable_fb_clear in vmx_update_fb_clear_dis(),
> it still refers to X86_FEATURE_CLEAR_CPU_BUF, is that wrong?

It looks correct to me. The only reason X86_FEATURE_CLEAR_CPU_BUF is used
in vmx_update_fb_clear_dis() is to check if host has enabled its
exit-to-userspace mitigation for some reason, and allow guest to also use
VERW.