linux-kernel - Re: [PATCH v3 64/75] x86/sev-es: Cache CPUID results for improved performance

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Message-ID: <cab5e409-be9e-f2c7-424f-6b7d031324dc@amd.com>
Date:   Wed, 27 May 2020 12:49:06 -0500
From:   Tom Lendacky <thomas.lendacky@....com>
To:     Borislav Petkov <bp@...en8.de>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Joerg Roedel <joro@...tes.org>, x86@...nel.org, hpa@...or.com,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Hellstrom <thellstrom@...are.com>,
        Jiri Slaby <jslaby@...e.cz>,
        Dan Williams <dan.j.williams@...el.com>,
        Juergen Gross <jgross@...e.com>,
        Kees Cook <keescook@...omium.org>,
        David Rientjes <rientjes@...gle.com>,
        Cfir Cohen <cfir@...gle.com>,
        Erdem Aktas <erdemaktas@...gle.com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mike Stunes <mstunes@...are.com>,
        Joerg Roedel <jroedel@...e.de>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH v3 64/75] x86/sev-es: Cache CPUID results for improved
 performance

On 5/26/20 4:19 AM, Borislav Petkov wrote:
> On Tue, May 19, 2020 at 10:16:37PM -0700, Sean Christopherson wrote:
>> The whole cache on-demand approach seems like overkill.  The number of CPUID
>> leaves that are invoked after boot with any regularity can probably be counted
>> on one hand.   IIRC glibc invokes CPUID to gather TLB/cache info, XCR0-based
>> features, and one or two other leafs.  A statically sized global array that's
>> arbitrarily index a la x86_capability would be just as simple and more
>> performant.  It would also allow fancier things like emulating CPUID 0xD in
>> the guest if you want to go down that road.
> 
> And before we do any of that "caching" or whatnot, I'd like to see
> numbers justifying its existence. Because if it is only a couple of
> CPUID invocations and the boot delay is immeasurable, then it's not
> worth the effort.

I added some rudimentary stats code to see how many times there was a 
CPUID cache hit on a 64-vCPU guest during a kernel build (make clean 
followed by make with -j 64):

   SEV-ES CPUID cache statistics
     0x00000000/0x00000000: 220,384
     0x00000007/0x00000000: 213,306
     0x80000000/0x00000000: 1,054,642
     0x80000001/0x00000000: 213,306
     0x80000005/0x00000000: 210,334
     0x80000006/0x00000000: 420,668
     0x80000007/0x00000000: 210,334
     0x80000008/0x00000000: 420,684

     2,963,658 cache hits

So it is significant in quantity, but I'm not sure what the overall 
performance difference is. If I can find some more time I'll try to 
compare the kernel builds with and without the caching to see if it is 
noticeable.

Thanks,
Tom

>