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

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Date:   Wed, 15 Dec 2021 10:53:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        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()

On Wed, Dec 15 2021 at 05:46, Kevin Tian wrote:
>> From: Thomas Gleixner <tglx@...utronix.de>
>> +	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.

I added it for paranoia sake because the destruction of the KVM FPU
state is not necessarily in the context of the vCPU thread. Yes, it
should not happen...

>> +	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.

Indeed.

Thanks,

        tglx