Re: [PATCH v5 1/9] KVM: VMX: Use on-stack copy of @flags in __vmx_vcpu_run()

Uros Bizjak
2025-11-14T15:06:53Z

On 11/14/25 00:37, Sean Christopherson wrote:
> When testing for VMLAUNCH vs. VMRESUME, use the copy of @flags from the
> stack instead of first moving it to EBX, and then propagating
> VMX_RUN_VMRESUME to RFLAGS.CF (because RBX is clobbered with the guest
> value prior to the conditional branch to VMLAUNCH). Stashing information
> in RFLAGS is gross, especially with the writer and reader being bifurcated
> by yet more gnarly assembly code.
>
> Opportunistically drop the SHIFT macros as they existed purely to allow
> the VM-Enter flow to use Bit Test.
>
> Suggested-by: Borislav Petkov <bp@alien8.de>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
> arch/x86/kvm/vmx/run_flags.h | 10 +++-------
> arch/x86/kvm/vmx/vmenter.S | 13 ++++---------
> 2 files changed, 7 insertions(+), 16 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/run_flags.h b/arch/x86/kvm/vmx/run_flags.h
> index 2f20fb170def..6a87a12135fb 100644
> --- a/arch/x86/kvm/vmx/run_flags.h
> +++ b/arch/x86/kvm/vmx/run_flags.h
> @@ -2,12 +2,8 @@
> #ifndef __KVM_X86_VMX_RUN_FLAGS_H
> #define __KVM_X86_VMX_RUN_FLAGS_H
>
> -#define VMX_RUN_VMRESUME_SHIFT 0
> -#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT 1
> -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT 2
> -
> -#define VMX_RUN_VMRESUME BIT(VMX_RUN_VMRESUME_SHIFT)
> -#define VMX_RUN_SAVE_SPEC_CTRL BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT)
> -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT)
> +#define VMX_RUN_VMRESUME BIT(0)
> +#define VMX_RUN_SAVE_SPEC_CTRL BIT(1)
> +#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(2)
>
> #endif /* __KVM_X86_VMX_RUN_FLAGS_H */
> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S
> index 574159a84ee9..93cf2ca7919a 100644
> --- a/arch/x86/kvm/vmx/vmenter.S
> +++ b/arch/x86/kvm/vmx/vmenter.S
> @@ -92,7 +92,7 @@ SYM_FUNC_START(__vmx_vcpu_run)
> /* Save @vmx for SPEC_CTRL handling */
> push %_ASM_ARG1
>
> - /* Save @flags for SPEC_CTRL handling */
> + /* Save @flags (used for VMLAUNCH vs. VMRESUME and mitigations). */
> push %_ASM_ARG3
>
> /*
> @@ -101,9 +101,6 @@ SYM_FUNC_START(__vmx_vcpu_run)
> */
> push %_ASM_ARG2
>
> - /* Copy @flags to EBX, _ASM_ARG3 is volatile. */
> - mov %_ASM_ARG3L, %ebx
> -
> lea (%_ASM_SP), %_ASM_ARG2
> call vmx_update_host_rsp
>
> @@ -147,9 +144,6 @@ SYM_FUNC_START(__vmx_vcpu_run)
> /* Load @regs to RAX. */
> mov (%_ASM_SP), %_ASM_AX
>
> - /* Check if vmlaunch or vmresume is needed */
> - bt $VMX_RUN_VMRESUME_SHIFT, %ebx
> -
> /* Load guest registers. Don't clobber flags. */
> mov VCPU_RCX(%_ASM_AX), %_ASM_CX
> mov VCPU_RDX(%_ASM_AX), %_ASM_DX
> @@ -173,8 +167,9 @@ SYM_FUNC_START(__vmx_vcpu_run)
> /* Clobbers EFLAGS.ZF */
> CLEAR_CPU_BUFFERS
>
> - /* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */
> - jnc .Lvmlaunch
> + /* Check @flags to see if vmlaunch or vmresume is needed. */
> + testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP)
> + jz .Lvmlaunch

You could use TESTB instead of TESTL in the above code to save 3 bytes
of code and some memory bandwidth.

Assembler will report unwanted truncation if VMX_RUN_VRESUME ever
becomes larger than 255.

BR,
Uros.