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@xxxxxxxxx> > > > Cc: Sean Christopherson <seanjc@xxxxxxxxxx> > > > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > > > Cc: Borislav Petkov <bp@xxxxxxxxx> > > > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > > > --- > > > 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.
