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

Tom Lendacky
2020-05-27T17:49:06Z

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

>