From: Andrew Cooper Date: Mon, 10 Feb 2020 11:27:32 +0000 (+0000) Subject: x86/svm: Reduce vmentry latency X-Git-Tag: archive/raspbian/4.14.0+80-gd101b417b7-1+rpi1^2~63^2~718 X-Git-Url: https://dgit.raspbian.org/?a=commitdiff_plain;h=640330d41e83af8f1b6fbe09a91712e50c411616;p=xen.git 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 Reviewed-by: Roger Pau Monné Acked-by: Jan Beulich --- 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