x86/svm: Reduce vmentry latency

[Xen-devel] [PATCH] x86/svm: Reduce vmentry latency

Posted by Andrew Cooper 4 years, 2 months ago

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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Re: [Xen-devel] [PATCH] x86/svm: Reduce vmentry latency

Posted by Roger Pau Monné 4 years, 2 months ago

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.

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Re: [Xen-devel] [PATCH] x86/svm: Reduce vmentry latency

Posted by Jan Beulich 4 years, 2 months ago

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>


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