Re: XSAVE / RDPKRU on Intel 11th Gen Core CPUs

[Brian Geffon]

Hi Dave,

On Tue, Nov 9, 2021 at 1:49 AM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
> Well, gosh, it's making it back to the software init value.  If you do:
>
>         echo 0x15555554 > /sys/kernel/debug/x86/init_pkru
>
> do you end up with 0x15555554 as the value?

What's interesting is that writing to init_pkru fails with -EINVAL for me,
and I've traced it down to get_xsave_addr() returning NULL on the following
check:

  /*
  * This assumes the last 'xsave*' instruction to
  * have requested that 'xfeature_nr' be saved.
  * If it did not, we might be seeing and old value
  * of the field in the buffer.
  *
  * This can happen because the last 'xsave' did not
  * request that this feature be saved (unlikely)
  * or because the "init optimization" caused it
  * to not be saved.
  */
  if (!(xsave->header.xfeatures & BIT_ULL(xfeature_nr)))
     return NULL;

And that's why I thought this was possibly related to that erratum
that I shared before.

> Does your system have any more XSAVE support than mine?
>
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating point registers'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x020: 'AVX-512 opmask'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x040: 'AVX-512 Hi256'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x080: 'AVX-512 ZMM_Hi256'
> > kernel: [    0.000000] x86/fpu: Supporting XSAVE feature 0x200: 'Protection Keys User registers'
> > kernel: [    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
> > kernel: [    0.000000] x86/fpu: xstate_offset[5]:  832, xstate_sizes[5]:   64
> > kernel: [    0.000000] x86/fpu: xstate_offset[6]:  896, xstate_sizes[6]:  512
> > kernel: [    0.000000] x86/fpu: xstate_offset[7]: 1408, xstate_sizes[7]: 1024
> > kernel: [    0.000000] x86/fpu: xstate_offset[9]: 2432, xstate_sizes[9]:    8
> > kernel: [    0.000000] x86/fpu: Enabled xstate features 0x2e7, context size is 2440 bytes, using 'compacted' format.

No, it's pretty much identical

INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87
floating point registers'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE
registers'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX
registers'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x020:
'AVX-512 opmask'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x040:
'AVX-512 Hi256'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x080:
'AVX-512 ZMM_Hi256'
INFO kernel: [ 0.000000] x86/fpu: Supporting XSAVE feature 0x200:
'Protection Keys User registers'
INFO kernel: [ 0.000000] x86/fpu: xstate_offset[2]: 576, xstate_sizes[2]: 256
INFO kernel: [ 0.000000] x86/fpu: xstate_offset[5]: 832, xstate_sizes[5]: 64
INFO kernel: [ 0.000000] x86/fpu: xstate_offset[6]: 896, xstate_sizes[6]: 512
INFO kernel: [ 0.000000] x86/fpu: xstate_offset[7]: 1408, xstate_sizes[7]: 1024
INFO kernel: [ 0.000000] x86/fpu: xstate_offset[9]: 2432, xstate_sizes[9]: 8
INFO kernel: [ 0.000000] x86/fpu: Enabled xstate features 0x2e7,
context size is 2440 bytes, using 'compacted' format.