Re: [Qemu-devel] [PATCH uq/master 1/2] x86: fix migration from pre-version 12

[Paolo Bonzini]

Il 09/09/2013 11:03, Gleb Natapov ha scritto:
> 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.

Yes, old->new migration can happen.  New->old of course cannot.

>> 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.

Because otherwise it behaves in a subtly different manner for XSAVE and
non-XSAVE hosts.

>> 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
the CPUID bits so that a well-behaved guest will not use it.  QEMU does
hide CPUID bits for non-supported XSAVE states, except for "-cpu host".
 So this will not be a problem except with "-cpu host".

Paolo