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

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Date:   Thu, 16 Dec 2021 01:00:33 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "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: Paolo Bonzini
> Sent: Wednesday, December 15, 2021 6:41 PM
> 
> On 12/15/21 11:27, Paolo Bonzini wrote:
> > On 12/15/21 11:09, Thomas Gleixner wrote:
> >> Lets assume the restore order is XSTATE, XCR0, XFD:
> >>
> >>       XSTATE has everything in init state, which means the default
> >>       buffer is good enough
> >>
> >>       XCR0 has everything enabled including AMX, so the buffer is
> >>       expanded
> >>
> >>       XFD has AMX disable set, which means the buffer expansion was
> >>       pointless
> >>
> >> If we go there, then we can just use a full expanded buffer for KVM
> >> unconditionally and be done with it. That spares a lot of code.
> >
> > If we decide to use a full expanded buffer as soon as KVM_SET_CPUID2 is
> > done, that would work for me.
> 
> Off-list, Thomas mentioned doing it even at vCPU creation as long as the
> prctl has been called.  That is also okay and even simpler.

Make sense. It also avoids the #GP thing in the emulation path if just due
to reallocation error.

We'll follow this direction to do a quick update/test. 

> 
> There's also another important thing that hasn't been mentioned so far:
> KVM_GET_SUPPORTED_CPUID should _not_ include the dynamic bits in
> CPUID[0xD] if they have not been requested with prctl.  It's okay to
> return the AMX bit, but not the bit in CPUID[0xD].
> 

will do.

Thanks
Kevin