[Xen-changelog] [xen staging] x86/svm: Reduce vmentry latency

[patchbot@xxxxxxx]

commit 640330d41e83af8f1b6fbe09a91712e50c411616
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Feb 10 11:27:32 2020 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Mon Feb 10 17:42:14 2020 +0000

    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@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hvm/svm/entry.S | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/hvm/svm/entry.S b/xen/arch/x86/hvm/svm/entry.S
index e954d8e021..1d2df08e89 100644
--- 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
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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