[Bug 1524069] Re: [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel exception]

[William Grant]

If you need avx2 support, --cpu Haswell-noTSX,-x2apic works on Haswell
desktop/laptop chips.

The most confusing problem is that qemu's definition of "Haswell" is
actually Haswell-E, -EP and -EX; Haswell itself lacks x2apic, which
qemu's Haswell requires. x2apic dates back to Nehalem, but qemu's CPU
definitions only include it back to Sandy Bridge, so a standard desktop
or laptop Haswell CPU falls all the back to Westmere and then adds flags
including avx, avx2 and xsave on top of that[0].

Advertising support for AVX and AVX2 is just a matter of setting
CPUID.1:ECX.AVX and CPUID.7:EBX.AVX2, but the instructions won't
actually work unless XCR0.AVX is set, and kvm_load_guest_xcr0 only sets
XCR0 from the guest if the guest's XR4.OSXSAVE is set. The guest's
fpu__init_cpu_xstate only sets CR4.OSXSAVE when xfeatures_mask is non-
zero, and xfeatures_mask is calculated by fpu__init_system_xstate from
CPUID.(EAX=0DH,ECX=0), which doesn't exist on qemu's Westmere
(level=0xb), so XCR0.AVX remains unset and AVX2 instructions #UD.

The bug is probably that raid6_have_avx2 only checks that AVX2 is
supported in CPUID, not that it's enabled. Checking for
X86_FEATURE_OSXSAVE might work, though I'm not sure if the value checked
by boot_cpu_has is stored too early for that.

I am also a little suspicious of kvm_load_guest_xcr0's CR4.OSXSAVE
guard. The Intel manuals state that XSAVE, XSRSTOR, XGETBV and XSETBV
require the flag to be set, but KVM won't restore a guest's existing
XCR0 unless OSXSAVE is still set.


[0] "-cpu Westmere,+invpcid,+erms,+bmi2,+smep,+avx2,+bmi1,+fsgsbase,+abm,+rdtscp,+pdpe1gb,+rdrand,+f16c,+avx,+osxsave,+xsave,+tsc-deadline,+movbe,+pcid,+pdcm,+xtpr,+fma,+tm2,+est,+vmx,+ds_cpl,+monitor,+dtes64,+pclmuldq,+pbe,+tm,+ht,+ss,+acpi,+ds,+vme"

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https://bugs.launchpad.net/bugs/1524069

Title:
  [Xenial] KVM trusty guest 3.13.0-68 raid6-pq panic in
  raid6_avx21_gen_syndrome() while probing grub devices [was: Xenial
  KVM: updating Trusty guest from 3.13.0-68 to 3.13.0-71 causes kernel
  exception]

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