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

[Gleb Natapov]

On Mon, Sep 09, 2013 at 01:54:50PM +0300, Gleb Natapov wrote:
> 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.
> 
BTW migration question, will xstate_bv no be zeroed by migration code in
old->new case?
 
--
                        Gleb.