RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

[Wang, Wei W]

Hi Thomas,

On Wednesday, December 15, 2021 5:36 AM, Juan Quintela wrote:
> To: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Wang, Wei W <wei.w.wang@xxxxxxxxx>; LKML
> <linux-kernel@xxxxxxxxxxxxxxx>; Dr. David Alan Gilbert <dgilbert@xxxxxxxxxx>;
> Jing Liu <jing2.liu@xxxxxxxxxxxxxxx>; Zhong, Yang <yang.zhong@xxxxxxxxx>;
> Paolo Bonzini <pbonzini@xxxxxxxxxx>; x86@xxxxxxxxxx; kvm@xxxxxxxxxxxxxxx;
> Sean Christoperson <seanjc@xxxxxxxxxx>; Nakajima, Jun
> <jun.nakajima@xxxxxxxxx>; Tian, Kevin <kevin.tian@xxxxxxxxx>
> Subject: Re: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()
> 
> Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> 
> Hi Thomas
> 
> > On Tue, Dec 14 2021 at 20:07, Juan Quintela wrote:
> >> Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >>> On Tue, Dec 14 2021 at 16:11, Wei W. Wang wrote:
> >>>> We need to check with the QEMU migration maintainer (Dave and Juan
> >>>> CC-ed) if changing that ordering would be OK.
> >>>> (In general, I think there are no hard rules documented for this
> >>>> ordering)
> >>>
> >>> There haven't been ordering requirements so far, but with dynamic
> >>> feature enablement there are.
> >>>
> >>> I really want to avoid going to the point to deduce it from the
> >>> xstate:xfeatures bitmap, which is just backwards and Qemu has all
> >>> the required information already.
> >>
> >> First of all, I claim ZERO knowledge about low level x86_64.
> >
> > Lucky you.
> 
> Well, that is true until I have to debug some bug, at that time I miss the
> knowledge O:-)
> 
> >> Once told that, this don't matter for qemu migration, code is at
> >
> > Once, that was at the time where rubber boots were still made of wood,
> > right? :)
> 
> I forgot to add: "famous last words".
> 
> >> target/i386/kvm/kvm.c:kvm_arch_put_registers()
> >>
> >>
> >>     ret = kvm_put_xsave(x86_cpu);
> >>     if (ret < 0) {
> >>         return ret;
> >>     }
> >>     ret = kvm_put_xcrs(x86_cpu);
> >>     if (ret < 0) {
> >>         return ret;
> >>     }
> >>     /* must be before kvm_put_msrs */
> >>     ret = kvm_inject_mce_oldstyle(x86_cpu);
> >
> > So this has already ordering requirements.
> >
> >>     if (ret < 0) {
> >>         return ret;
> >>     }
> >>     ret = kvm_put_msrs(x86_cpu, level);
> >>     if (ret < 0) {
> >>         return ret;
> >>     }
> >>
> >> If it needs to be done in any other order, it is completely
> >> independent of whatever is inside the migration stream.
> >
> > From the migration data perspective that's correct, but I have the
> > nagging feeling that this in not that simple.
> 
> Oh, I was not meaning that it was simple at all.

It seems to be a consensus that the ordering constraint wouldn't be that easy.
Would you think that our current solution (the 3 parts shared earlier to do fpstate expansion at KVM_SET_XSAVE) is acceptable as the 1st version?

Thanks,
Wei