Re: [PATCH] KVM: VMX: Micro-optimize SPEC_CTRL handling in __vmx_vcpu_run()

[Sean Christopherson <seanjc@google.com>]

On Tue, Aug 19, 2025, Uros Bizjak wrote:
> > >   2d: 48 8b 7c 24 10          mov    0x10(%rsp),%rdi
> > >   32: 8b 87 48 18 00 00       mov    0x1848(%rdi),%eax
> > >   38: 65 3b 05 00 00 00 00    cmp    %gs:0x0(%rip),%eax
> > >   3f: 74 09                   je     4a <...>
> > >   41: b9 48 00 00 00          mov    $0x48,%ecx
> > >   46: 31 d2                   xor    %edx,%edx
> > >   48: 0f 30                   wrmsr
> > >
> > > No functional change intended.
> > >
> > > Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
> > > Cc: Sean Christopherson <seanjc@google.com>
> > > Cc: Paolo Bonzini <pbonzini@redhat.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: Borislav Petkov <bp@alien8.de>
> > > Cc: Dave Hansen <dave.hansen@linux.intel.com>
> > > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > > ---
> > >  arch/x86/kvm/vmx/vmenter.S | 6 ++----
> > >  1 file changed, 2 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> > > index 0a6cf5bff2aa..c65de5de92ab 100644
> > > --- a/arch/x86/kvm/vmx/vmenter.S
> > > +++ b/arch/x86/kvm/vmx/vmenter.S
> > > @@ -118,13 +118,11 @@ SYM_FUNC_START(__vmx_vcpu_run)
> > >        * and vmentry.
> > >        */
> > >       mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI
> > > -     movl VMX_spec_ctrl(%_ASM_DI), %edi
> > > -     movl PER_CPU_VAR(x86_spec_ctrl_current), %esi
> > > -     cmp %edi, %esi
> > > +     movl VMX_spec_ctrl(%_ASM_DI), %eax
> > > +     cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax
> >
> > Huh.  There's a pre-existing bug lurking here, and in the SVM code.  SPEC_CTRL
> > is an MSR, i.e. a 64-bit value, but the assembly code assumes bits 63:32 are always
> > zero.
> 
> But MSBs are zero, MSR is defined in arch/x86/include/msr-index.h as:
> 
> #define MSR_IA32_SPEC_CTRL 0x00000048 /* Speculation Control */
> 
> and "movl $..., %eax" zero-extends the value to full 64-bit width.
> 
> FWIW, MSR_IA32_SPEC_CTR is handled in the same way in arch/x86/entry/entry.S:
> 
> movl $MSR_IA32_PRED_CMD, %ecx

That's the MSR index, not the value.  I'm pointing out that:

	movl VMX_spec_ctrl(%_ASM_DI), %edi              <== drops vmx->spec_ctrl[63:32]
	movl PER_CPU_VAR(x86_spec_ctrl_current), %esi   <== drop x86_spec_ctrl_current[63:32]
	cmp %edi, %esi                                  <== can get false negatives
	je .Lspec_ctrl_done
	mov $MSR_IA32_SPEC_CTRL, %ecx
	xor %edx, %edx                                  <== can clobber guest value
	mov %edi, %eax
	wrmsr

The bug is _currently_ benign because neither KVM nor the kernel support setting
any of bits 63:32, but it's still a bug that needs to be fixed.