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, and 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 <[email protected]>
---
CC: Jan Beulich <[email protected]>
CC: Roger Pau Monné <[email protected]>
CC: Wei Liu <[email protected]>
Zen3 doesn't get a speedup in general, because we use the WRMSR's to clear
IBRS to avoid forcing it behind a VM's back.
---
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 0ff4008060fa..a60d759f7108 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. */
--
2.11.0
