[xen master] x86/spec-ctrl: Make svm_vmexit_spec_ctrl conditional

[patchbot@xxxxxxx]

commit c16a9eda77b2089206d5bc39ab6488c3793e11bf
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Jul 18 14:15:08 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Jul 19 14:28:18 2022 +0100

    x86/spec-ctrl: Make svm_vmexit_spec_ctrl conditional
    
    The logic was written this way out of an abundance of caution, but the 
reality
    is that AMD parts don't currently have the RAS-flushing side effect, nor do
    they intend to gain it.
    
    This removes one WRMSR from the VMExit path by default on Zen2 systems.
    
    Fixes: 614cec7d79d7 ("x86/svm: VMEntry/Exit logic for MSR_SPEC_CTRL")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/svm/entry.S | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index 0ff4008060..a60d759f71 100644
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -113,15 +113,15 @@ __UNLIKELY_END(nsvm_hap)
         ALTERNATIVE "", DO_OVERWRITE_RSB, X86_FEATURE_SC_RSB_HVM
 
         .macro svm_vmexit_spec_ctrl
-            /*
-             * Write to MSR_SPEC_CTRL unconditionally, for the RAS[:32]
-             * flushing side effect.
-             */
-            mov    $MSR_SPEC_CTRL, %ecx
             movzbl CPUINFO_xen_spec_ctrl(%rsp), %eax
+            movzbl CPUINFO_last_spec_ctrl(%rsp), %edx
+            cmp    %edx, %eax
+            je     1f /* Skip write if value is correct. */
+            mov    $MSR_SPEC_CTRL, %ecx
             xor    %edx, %edx
             wrmsr
             mov    %al, CPUINFO_last_spec_ctrl(%rsp)
+1:
         .endm
         ALTERNATIVE "", svm_vmexit_spec_ctrl, X86_FEATURE_SC_MSR_HVM
         /* WARNING! `ret`, `call *`, `jmp *` not safe before this point. */
--
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