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Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-versi
From: |
Gleb Natapov |
Subject: |
Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12 |
Date: |
Mon, 9 Sep 2013 12:03:15 +0300 |
On Mon, Sep 09, 2013 at 10:31:15AM +0200, Paolo Bonzini wrote:
> Il 08/09/2013 13:40, Gleb Natapov ha scritto:
> > On Thu, Sep 05, 2013 at 03:06:21PM +0200, Paolo Bonzini wrote:
> >> On KVM, the KVM_SET_XSAVE would be executed with a 0 xstate_bv,
> >> and not restore anything.
> >>
> > XRSTOR restores FP/SSE state to reset state if no bits are set in
> > xstate_bv. This is what should happen on reset, no?
>
> Yes. The problem happens on the migration destination when XSAVE data is
> not transmitted. FP/SSE data is transmitted and must be restored, but
> xstate_bv is zero and KVM_SET_XSAVE restores FP/SSE state to reset
> state. The vcpu then loses the values that were set in the migration data.
>
> >> Since FP and SSE data are always valid, set them in xstate_bv at reset
> >> time. In fact, that value is the same that KVM_GET_XSAVE returns on
> >> pre-XSAVE hosts.
> > It is needed for migration between non xsave host to xsave host.
>
> Yes, and this patch does the same for migration between non-XSAVE QEMU
> and XSAVE QEMU.
>
Can such migration happen? The commit that added xsave support
(f1665b21f16c5dc0ac37de60233a4975aff31193) changed vmstate version id.
> In fact, another bug is that kvm_vcpu_ioctl_x86_set_xsave ignores
> xstate_bv when XSAVE is not available. Instead, it should reset the
> FXSAVE data to processor-reset values (except for MXCSR which always
> comes from XRSTOR data), i.e. to all-zeros except for the x87 control
> and tag words. It should also check reserved bits of MXCSR.
>
I do not see why.
> >>
> >> Signed-off-by: Paolo Bonzini <address@hidden>
> >> ---
> >> target-i386/cpu.c | 1 +
> >> target-i386/cpu.h | 5 +++++
> >> 2 files changed, 6 insertions(+)
> >>
> >> diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> >> index c36345e..ac83106 100644
> >> --- a/target-i386/cpu.c
> >> +++ b/target-i386/cpu.c
> >> @@ -2386,6 +2386,7 @@ static void x86_cpu_reset(CPUState *s)
> >> env->fpuc = 0x37f;
> >>
> >> env->mxcsr = 0x1f80;
> >> + env->xstate_bv = XSTATE_FP | XSTATE_SSE;
> >>
> >> env->pat = 0x0007040600070406ULL;
> >> env->msr_ia32_misc_enable = MSR_IA32_MISC_ENABLE_DEFAULT;
> >> diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> >> index 5723eff..a153078 100644
> >> --- a/target-i386/cpu.h
> >> +++ b/target-i386/cpu.h
> >> @@ -380,6 +380,11 @@
> >>
> >> #define MSR_VM_HSAVE_PA 0xc0010117
> >>
> >> +#define XSTATE_SUPPORTED (XSTATE_FP|XSTATE_SSE|XSTATE_YMM)
> > Supported by whom? By QEMU? We should filer unsupported bits from CPUID.0D
> > then too.
>
> Yes. QEMU unmarshals information from the XSAVE region and back, so it
> cannot support MPX or AVX-512 yet (even if KVM were). Separate bug, though.
>
IMO this is the main issue here, not separate bug. If we gonna let guest
use CPU state QEMU does not support we gonna have a bad time.
--
Gleb.
- [Qemu-devel] [PATCH uq/master 0/2] KVM: issues with XSAVE support, Paolo Bonzini, 2013/09/05
- [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Paolo Bonzini, 2013/09/05
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Gleb Natapov, 2013/09/08
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12,
Gleb Natapov <=
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Gleb Natapov, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Gleb Natapov, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Gleb Natapov, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12, Gleb Natapov, 2013/09/09
[Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Paolo Bonzini, 2013/09/05