linux-kernel - RE: [patch 4/6] x86/fpu: Add guest support to xfd_enable

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Date:   Wed, 15 Dec 2021 05:46:53 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
CC:     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>
Subject: RE: [patch 4/6] x86/fpu: Add guest support to xfd_enable_feature()

> From: Thomas Gleixner <tglx@...utronix.de>
> Sent: Tuesday, December 14, 2021 9:16 PM
> 
> On Tue, Dec 14 2021 at 11:21, Paolo Bonzini wrote:
> > On 12/14/21 07:05, Tian, Kevin wrote:
> >>> +	if (guest_fpu) {
> >>> +		newfps->is_guest = true;
> >>> +		newfps->is_confidential = curfps->is_confidential;
> >>> +		newfps->in_use = curfps->in_use;
> >>> +		guest_fpu->xfeatures |= xfeatures;
> >>> +	}
> >>> +
> >> As you explained guest fpstate is not current active in the restoring
> >> path, thus it's not correct to always inherit attributes from the
> >> active one.
> 
> Good catch!
> 
> > Indeed, guest_fpu->fpstate should be used instead of curfps.
> 
> Something like the below.

Looks good. Just two nits.

> +	/*
> +	 * When a guest FPU is supplied, use @guest_fpu->fpstate
> +	 * as reference independent whether it is in use or not.
> +	 */
> +	curfps = guest_fpu ? guest_fpu->fpstate : fpu->fpstate;
> +
> +	/* Determine whether @curfps is the active fpstate */
> +	in_use = fpu->fpstate == curfps;
> +
> +	if (guest_fpu) {
> +		newfps->is_guest = true;
> +		newfps->is_confidential = curfps->is_confidential;
> +		newfps->in_use = curfps->in_use;

What is the purpose of this 'in_use' field? Currently it's only
touched in three places:

  - set when entering guest;
  - cleared when exiting to userspace;
  - checked when freeing a guest FPU;

The last one can be easily checked by comparing to current fps.

Any other intended usage which is currently missed for guest FPU?

> 
> +	if (guest_fpu) {
> +		curfps = xchg(&guest_fpu->fpstate, newfps);

This can be a direct value update to guest_fpu->fpstate since 
curfps has already been acquired in the start.

> +		/* If curfps is active, update the FPU fpstate pointer */
> +		if (in_use)
> +			fpu->fpstate = newfps;
> +	} else {
> +		curfps = xchg(&fpu->fpstate, newfps);

ditto.

Thanks
Kevin