RE: [patch 4/6] x86/fpu: Add guest support to xfd_enable_feature()

[Thomas Gleixner <tglx@linutronix.de>]

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