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>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
In a small test where I wired ICEBP to tighly re-enter the guest, this dropped
the guests perviced time for ICEBP (as close to one vmexit and entry as I
could realistically manage) by 20 ticks. Sadly, that also seems to be the
granuarlity of measurement. The modal measurement (accounting for 80% of
samples) was 1200 ticks, and reduced to 1180 with just this change in place.
---
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
--
2.11.0
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On Mon, Feb 10, 2020 at 11:42:06AM +0000, Andrew Cooper wrote: > 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> Thanks. _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
On 10.02.2020 13:09, Roger Pau Monné wrote: > On Mon, Feb 10, 2020 at 11:42:06AM +0000, Andrew Cooper wrote: >> 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> _______________________________________________ Xen-devel mailing list Xen-devel@lists.xenproject.org https://lists.xenproject.org/mailman/listinfo/xen-devel
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