linux-kernel - RE: [PATCH 1/2] x86/fpu: Measure the Latency of XSAVE and XRSTOR

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Sun, 24 Jul 2022 20:54:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Yi Sun' <yi.sun@...el.com>,
        "dave.hansen@...el.com" <dave.hansen@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>
CC:     "sohil.mehta@...el.com" <sohil.mehta@...el.com>,
        "tony.luck@...el.com" <tony.luck@...el.com>,
        "heng.su@...el.com" <heng.su@...el.com>
Subject: RE: [PATCH 1/2] x86/fpu: Measure the Latency of XSAVE and XRSTOR

From: Yi Sun
> Sent: 23 July 2022 09:38
> 
> Calculate the latency of instructions xsave and xrstor with new trace
> points x86_fpu_latency_xsave and x86_fpu_latency_xrstor.
> 
> The delta TSC can be calculated within a single trace event. Another
> option considered was to have 2 separated trace events marking the
> start and finish of the xsave/xrstor instructions. The delta TSC was
> calculated from the 2 trace points in user space, but there was
> significant overhead added by the trace function itself.
> 
> In internal testing, the single trace point option which is
> implemented here proved to be more accurate.
...

I've done some experiments that measure short instruction latencies.
Basically I found:
1) You need a suitable serialising instruction before and after
   the code being tested - otherwise it can overlap whatever
   you are using for timing.
2) The only reliable counter is the performance monitor clock
   counter - everything else depends on the current cpu frequency.
   On intel cpu the cpu frequency can change all the time.
Allowing for that, and then ignoring complete outliers, I could
get clock-count accurate values for iterations of the IP csum loop.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)