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

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Date:   Tue, 14 Dec 2021 19:04:50 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Wang, Wei W" <wei.w.wang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Dr. David Alan Gilbert" <dgilbert@...hat.com>,
        Juan Quintela <quintela@...hat.com>
Cc:     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()

Wei,

On Tue, Dec 14 2021 at 16:11, Wei W. Wang wrote:
> On Tuesday, December 14, 2021 11:40 PM, Thomas Gleixner wrote:
>> On Tue, Dec 14 2021 at 15:09, Wei W. Wang wrote:
>> > On Tuesday, December 14, 2021 10:50 AM, Thomas Gleixner wrote:
>> >> + * Return: 0 on success, error code otherwise  */ int
>> >> +__fpu_update_guest_features(struct fpu_guest *guest_fpu, u64 xcr0,
>> >> +u64
>> >> +xfd) {
>> >
>> > I think there would be one issue for the "host write on restore" case.
>> > The current QEMU based host restore uses the following sequence:
>> > 1) restore xsave
>> > 2) restore xcr0
>> > 3) restore XFD MSR
>> 
>> This needs to be fixed. Ordering clearly needs to be:
>> 
>>   XFD, XCR0, XSTATE
>
> Sorry, just to clarify that the ordering in QEMU isn't made by us for this specific XFD enabling.
> It has been there for long time for the general restoring of all the XCRs and MSRs.
> (if you are interested..FYI: https://github.com/qemu/qemu/blob/master/target/i386/kvm/kvm.c#L4168).
> - kvm_put_xsave()
> - kvm_put_xcrs()
> - kvm_put_msrs()
>
> 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.

Thanks,

        tglx