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

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Date:   Fri, 17 Dec 2021 15:33:34 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "quintela@...hat.com" <quintela@...hat.com>
CC:     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>,
        "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "Zeng, Guang" <guang.zeng@...el.com>
Subject: RE: [patch 5/6] x86/fpu: Provide fpu_update_guest_xcr0/xfd()

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Thursday, December 16, 2021 10:13 PM
> >
> > Yes, this is the 3rd open that I asked in another reply. The only restriction
> > with this approach is that the sync cost is added also for legacy OS which
> > doesn't touch xfd at all.
> 
> You still can make that conditional on the guest XCR0. If guest never
> enables the extended bit then neither the #NM trap nor the XFD sync
> are required.
> 
> But yes, there are too many moving parts here :)
> 

Yes. Many moving parts but in general it's getting cleaner and simplified. 😊

We just sent out v2 to hopefully lock down already-closed opens. Based on
that we can see what remains to be further solved.

And really appreciate all the suggestions from you and Paolo!

Thanks
Kevin