linux-kernel - Re: [patch 5/6] x86/fpu: Provide fpu_update_guest

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Message-ID: <87k0g7rkwj.ffs@tglx>
Date:   Tue, 14 Dec 2021 21:28:28 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     quintela@...hat.com
Cc:     "Wang, Wei W" <wei.w.wang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "Zhong, Yang" <yang.zhong@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Sean Christoperson <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>
Subject: Re: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

Juan,

On Tue, Dec 14 2021 at 20:07, Juan Quintela wrote:
> Thomas Gleixner <tglx@...utronix.de> 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.

> 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? :)

> 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.

> I guess that Paolo will put some light here.

I fear shining light on that will unearth quite a few skeletons :)

Thanks,

        tglx