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Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-versi
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12 |
Date: |
Mon, 09 Sep 2013 13:46:49 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130805 Thunderbird/17.0.8 |
Il 09/09/2013 13:28, Gleb Natapov ha scritto:
>> On an XSAVE host, when the guest FPU state is loaded KVM will do an
>> XRSTOR. The XRSTOR will restore the FPU state to default values.
>>
>> On a non-XSAVE host, when the guest FPU state is loaded KVM will do an
>> FXRSTR. The FXRSTR will load the FPU state from the first 512 bytes of
>> the block that was passed to KVM_SET_XSAVE.
>>
>> This is not a problem because userspace will usually pass to
>> KVM_SET_XSAVE only something that it got from KVM_GET_XSAVE, and
>> KVM_GET_XSAVE will never set XSTATE_BV=0. However, KVM_SET_XSAVE is
>> supposed to emulate XSAVE/XRSTOR if it is not available, and it is
>> failing to emulate this detail.
>>
> You are trying to be bug to bug compatible :) XSTATE_BV can be zero only
> if FPU state is reset one, otherwise the guest will not survive.
Yes.
> KVM_SET_XSAVE
> is not suppose to emulate XSAVE/XRSTOR, it is not emulator function. It
> is better to outlaw zero value for XSTATE_BV at all, but we cannot do it
> because current QEMU uses it.
I agree it'd be better to forbid it. If the mismatch in semantics does
not bother you, I won't fix it. It slightly bothers me. :)
>>>>>> 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.
>>>>
>>>> We cannot force the guest not to use a feature; all we can do is hide
>>>
>>> Of course we can't, this is correct for other features too, but this is
>>> guest's problem.
>>
>> Ok, then we agree that QEMU doesn't have a problem? The XSAVE data will
>
> Which problem exactly. The problems I see is that 1. We do not support
> MPX and AVX-512 (but this is probably not the problem you meant :)) 2. 0D
> data is not consistent with features. Guest may not expect it and do stupid
> things.
It is not a problem to unmarshal information out of KVM_GET_XSAVE data
(and back). If the guest does stupid things, it's a bug in an
ill-behaving guest.
On the other hand, I agree that passthrough of host 0xD data is bad and
will fix it.
Paolo
>> always be "fresh" as long as the guest obeys CPUID bits it receives, and
>> the CPUID bits that QEMU passes will never enable XSAVE data that QEMU
>> does not support.
>>
>
> --
> 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, 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, 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 <=
- 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
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Gleb Natapov, 2013/09/08
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Gleb Natapov, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Paolo Bonzini, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Gleb Natapov, 2013/09/09
- Re: [Qemu-devel] [PATCH uq/master 2/2] KVM: make XSAVE support more robust, Paolo Bonzini, 2013/09/09