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

[Gleb Natapov]

On Mon, Sep 09, 2013 at 11:53:45AM +0200, Paolo Bonzini wrote:
> 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.
> 
I see. I am fine with the patch, but please drop defines that are not
used in the patch itself.

> >> 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.
I do not see how. Can you elaborate?

> 
> >> 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.

> 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".
> 

--
                        Gleb.