linux-kernel - RE: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

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Date:   Fri, 15 Oct 2021 09:20:13 +0000
From:   "Liu, Jing2" <jing2.liu@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Arjan van de Ven" <arjan@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        Jing Liu <jing2.liu@...ux.intel.com>,
        "seanjc@...gle.com" <seanjc@...gle.com>,
        "Cooper, Andrew" <andrew.cooper3@...rix.com>
Subject: RE: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core


On 10/15/2021 3:14 AM, Thomas Gleixner wrote:
> Paolo,
> 
[...]
> >> vcpu_create()
> >>
> >>    fpu_init_fpstate_user(guest_fpu, supported_xcr0)
> >>
> >> That will (it does not today) do:
> >>
> >>       guest_fpu::__state_perm = supported_xcr0 &
> >> xstate_get_group_perm();
> >>
> >> The you have the information you need right in the guest FPU.
> >
> > Good, I wasn't aware of the APIs that will be there.
> 
> Me neither, but that's a pretty obvious consequence of the work I'm doing
> for AMX. So I made it up for you. :)

Do you mean that fpu_init_fpstate_user() will be updated to add
supported_xcr0 later? :) 

I'm thinking if guest_fpu::xfd is good to directly initialize as user's
init_fpstate.xfd. Because before guest initializes XFD, "hardware"
is reset value. So it would be better to make guest_fpu::xfd the
same so no need to reload zero before vmenter during this time.

Thanks,
Jing