Pinging to bring attention to this fix.
On Wed, 2024-08-07 at 14:35 +0200, Amit Shah wrote:
> From: Amit Shah <amit.shah@xxxxxxx>
>
> Remove superfluous RSB filling after a VMEXIT when the CPU already
> has
> flushed the RSB after a VMEXIT when AutoIBRS is enabled.
>
> The initial implementation for adding RETPOLINES added an
> ALTERNATIVES
> implementation for filling the RSB after a VMEXIT in
>
> commit 117cc7a908c836 ("x86/retpoline: Fill return stack buffer on
> vmexit")
>
> Later, X86_FEATURE_RSB_VMEXIT was added in
>
> commit 2b129932201673 ("x86/speculation: Add RSB VM Exit
> protections")
>
> The AutoIBRS (on AMD CPUs) feature implementation added in
>
> commit e7862eda309ecf ("x86/cpu: Support AMD Automatic IBRS")
>
> used the already-implemented logic for EIBRS in
> spectre_v2_determine_rsb_fill_type_on_vmexit() -- but did not update
> the
> code at VMEXIT to act on the mode selected in that function --
> resulting
> in VMEXITs continuing to clear the RSB when RETPOLINES are enabled,
> despite the presence of AutoIBRS.
>
> Signed-off-by: Amit Shah <amit.shah@xxxxxxx>
>
> ---
> v4: resend of v3 with subject-prefix fixed
>
> v3:
> - Add a comment mentioning SVM does not need RSB_VMEXIT_LITE unlike
> VMX.
> v2:
> - tweak commit message re: Boris's comments.
>
> arch/x86/kvm/svm/vmenter.S | 24 ++++++++++++++++--------
> 1 file changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/vmenter.S b/arch/x86/kvm/svm/vmenter.S
> index a0c8eb37d3e1..69d9825ebdd9 100644
> --- a/arch/x86/kvm/svm/vmenter.S
> +++ b/arch/x86/kvm/svm/vmenter.S
> @@ -209,10 +209,14 @@ SYM_FUNC_START(__svm_vcpu_run)
> 7: vmload %_ASM_AX
> 8:
>
> -#ifdef CONFIG_MITIGATION_RETPOLINE
> - /* IMPORTANT: Stuff the RSB immediately after VM-Exit,
> before RET! */
> - FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS,
> X86_FEATURE_RETPOLINE
> -#endif
> + /*
> + * IMPORTANT: Stuff the RSB immediately after VM-Exit,
> before RET!
> + *
> + * Unlike VMX, AMD does not have the hardware bug that
> necessitates
> + * RSB_VMEXIT_LITE
> + */
> +
> + FILL_RETURN_BUFFER %_ASM_AX, RSB_CLEAR_LOOPS,
> X86_FEATURE_RSB_VMEXIT
>
> /* Clobbers RAX, RCX, RDX. */
> RESTORE_HOST_SPEC_CTRL
> @@ -348,10 +352,14 @@ SYM_FUNC_START(__svm_sev_es_vcpu_run)
>
> 2: cli
>
> -#ifdef CONFIG_MITIGATION_RETPOLINE
> - /* IMPORTANT: Stuff the RSB immediately after VM-Exit,
> before RET! */
> - FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS,
> X86_FEATURE_RETPOLINE
> -#endif
> + /*
> + * IMPORTANT: Stuff the RSB immediately after VM-Exit,
> before RET!
> + *
> + * Unlike VMX, AMD does not have the hardware bug that
> necessitates
> + * RSB_VMEXIT_LITE
> + */
> +
> + FILL_RETURN_BUFFER %rax, RSB_CLEAR_LOOPS,
> X86_FEATURE_RSB_VMEXIT
>
> /* Clobbers RAX, RCX, RDX, consumes RDI (@svm) and RSI
> (@spec_ctrl_intercepted). */
> RESTORE_HOST_SPEC_CTRL
