On Mon, Apr 16, 2012 at 5:50 AM, Joanna Rutkowska <
joa...@invisiblethingslab.com> wrote:
> On 04/16/12 14:20, Joanna Rutkowska wrote:
> /.../
> >
> > So, it seems like it is not a Xen issue, but instead that the kernel
> > I'm using in the VM (essentially a vanilla 3.0.4) is not enabling AVX
> > in XCR0. It would be interesting if anybody could try this on a
> > non-Xen system with a similarly old kernel as I have (and on the
> > AVX-capable processor, of course).
> >
>
> Interestingly, I tried another kernel in the VM (essentially a vanilla
> 3.2.7), and again the AVX seemed to be disabled in the VM (volk used
> sse4 again).
>
> So, I quickly looked through the kernel sources and I think that the
> kernel enables AVX in this code:
>
> static void __init xstate_enable_boot_cpu(void)
> {
> /.../
> cpuid_count(XSTATE_CPUID, 0, &eax, &ebx, &ecx, &edx);
> pcntxt_mask = eax + ((u64)edx << 32);
>
> if ((pcntxt_mask & XSTATE_FPSSE) != XSTATE_FPSSE) {
> printk(KERN_ERR "FP/SSE not shown under xsave features 0x%llx\n",
> pcntxt_mask);
> BUG();
> }
>
> /*
> * Support only the state known to OS.
> */
> pcntxt_mask = pcntxt_mask & XCNTXT_MASK;
>
> xstate_enable();
>
>
> The XSTATE_FPSSE and XCNTXT_MASK are defined as follows:
>
> #define XSTATE_FP 0x1
> #define XSTATE_SSE 0x2
> #define XSTATE_YMM 0x4
>
> #define XSTATE_FPSSE (XSTATE_FP | XSTATE_SSE)
> #define XCNTXT_MASK (XSTATE_FP | XSTATE_SSE | XSTATE_YMM)
>
> The YMM corresponds to the AVX enable flag (not sure why they call it
> differently?).
>
> Anyway, this shows that, assuming CPUID returns correct values (that
> indicate AVX to be enabled), then the (guest) kernel should enable AVX
> (and Xen should emulate this and allow for this, as indicated in the
> previous message). And if CPUID was not returning correct values (i.e.
> omitting the AVX flag), then we would not have this whole discussion.
>
> So, what am I missing?
>
> Can you point out which kernels do you use that you have AVX working fine?
>
Linux smidgen 3.0.0-17-generic #30-Ubuntu SMP Thu Mar 8 20:45:39 UTC 2012
x86_64 x86_64 x86_64 GNU/Linux
I really do think it's a Xen issue; do you have a kernel you can try to run
outside the hypervisor?
In any case, it's mostly a moot point, as there are very few Volk kernels
which make use of AVX, and those which do don't appear to be significantly
faster than their SSE3 counterparts. I think that's due to either memory
bandwidth or instruction promotion of SSE3 intrinsics, but that's a
different story.
--n
>
> joanna.
>
>
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