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    SubjectRe: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0 is loaded
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    On 15/03/2016 19:27, Andy Lutomirski wrote:
    > On Mon, Mar 14, 2016 at 6:17 AM, Paolo Bonzini <pbonzini@redhat.com> wrote:
    >>
    >>
    >> On 11/03/2016 22:33, David Matlack wrote:
    >>>> Is this better than just always keeping the host's XCR0 loaded outside
    >>>> if the KVM interrupts-disabled region?
    >>>
    >>> Probably not. AFAICT KVM does not rely on it being loaded outside that
    >>> region. xsetbv isn't insanely expensive, is it? Maybe to minimize the
    >>> time spent with interrupts disabled it was put outside.
    >>>
    >>> I do like that your solution would be contained to KVM.
    >>
    >> I agree with Andy. We do want a fix for recent kernels because of the
    >> !eager_fpu case that Guangrong mentioned.
    >>
    >> Paolo
    >>
    >> ps: while Andy is planning to kill lazy FPU, I want to benchmark it with
    >> KVM... Remember that with a single pre-xsave host in your cluster, your
    >> virt management might happily default your VMs to a Westmere or Nehalem
    >> CPU model. GCC might be a pretty good testbench for this (e.g. a kernel
    >> compile with very high make -j), because outside of the lexer (which
    >> plays SIMD games) it never uses the FPU.
    >
    > Aren't pre-xsave CPUs really, really old? A brief search suggests
    > that Intel Core added it somewhere in the middle of the cycle.

    I am fairly sure it was added in Sandy Bridge, together with AVX. But
    what really matters for eager FPU is not xsave, it's xsaveopt, and I
    think AMD has never even produced a microprocessor that supports it.

    > For pre-xsave, it could indeed hurt performance a tiny bit under
    > workloads that use the FPU and then stop completely because the
    > xsaveopt and init optimizations aren't available. But even that is
    > probably a very small effect, especially because pre-xsave CPUs have
    > smaller FPU state sizes.

    It's still a few cache lines. Benchmarks will tell.

    Paolo

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