x86/svm: Reduce vmentry latency

Writing to the stack pointer in the middle of a line of pop operations is
specifically recommended against by the optimisation guide, and is a technique
used by Speculative Load Hardening to combat SpectreRSB.

In practice, it causes all further stack-relative accesses to block until the
write to the stack pointer retires, so the stack engine can get back in sync.

Pop into any dead register to discard %rax's value without clobbering the
stack engine.  Smaller compiled code, and runs faster.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index e954d8e021104ea3943b21ada18004d56a8c85f6..1d2df08e897985cc7ad45455056c57870098c9f3 100644 (file)
--- a/xen/arch/x86/hvm/svm/entry.S
+++ b/xen/arch/x86/hvm/svm/entry.S
@@ -76,7 +76,7 @@ __UNLIKELY_END(nsvm_hap)
         pop  %r10
         pop  %r9
         pop  %r8
-        add  $8,%rsp /* Skip %rax: restored by VMRUN. */
+        pop  %rcx /* Skip %rax: restored by VMRUN. */
         pop  %rcx
         pop  %rdx
         pop  %rsi