Re: [PATCH v3 10/21] x86/fpu/xstate: Update xstate save function to support dynamic xstate

[Liu, Jing2]

On 1/15/2021 12:59 PM, Bae, Chang Seok wrote:
> > On Jan 11, 2021, at 18:52, Liu, Jing2 <jing2.liu@xxxxxxxxxxxxxxx> wrote:
> >
> > On 1/8/2021 2:40 AM, Bae, Chang Seok wrote:
> > > > On Jan 7, 2021, at 17:41, Liu, Jing2 <jing2.liu@xxxxxxxxx> wrote:
> > > >
> > > > static void kvm_save_current_fpu(struct fpu *fpu)  {
> > > > +	struct fpu *src_fpu = &current->thread.fpu;
> > > > +
> > > > 	/*
> > > > 	 * If the target FPU state is not resident in the CPU registers, just
> > > > 	 * memcpy() from current, else save CPU state directly to the target.
> > > > 	 */
> > > > -	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> > > > -		memcpy(&fpu->state, &current->thread.fpu.state,
> > > > +	if (test_thread_flag(TIF_NEED_FPU_LOAD)) {
> > > > +		memcpy(&fpu->state, &src_fpu->state,
> > > > 		       fpu_kernel_xstate_min_size);
> <snip>
>
> > > > -	else
> > > > +	} else {
> > > > +		if (fpu->state_mask != src_fpu->state_mask)
> > > > +			fpu->state_mask = src_fpu->state_mask;
> > > >
> > > > Though dynamic feature is not supported in kvm now, this function still need
> > > > consider more things for fpu->state_mask.
> > > Can you elaborate this? Which path might be affected by fpu->state_mask
> > > without dynamic state supported in KVM?
> > >
> > > > I suggest that we can set it before if...else (for both cases) and not change other.
> > > I tried a minimum change here.  The fpu->state_mask value does not impact the
> > > memcpy(). So, why do we need to change it for both?
> > Sure, what I'm considering is that "mask" is the first time introduced into "fpu",
> > representing the usage, so not only set it when needed, but also make it as a
> > representation, in case of anywhere using it (especially between the interval
> > of this series and kvm series in future).
> Thank you for the feedback. Sorry, I don't get any logical reason to set the
> mask always here.

Sure, I'd like to see if fx_init()->memset is the case,

though maybe no hurt so far in test.

Thanks,

Jing

>   Perhaps, KVM code can be updated like you mentioned when
> supporting the dynamic states there.
>
> Please let me know if I’m missing any functional issues.
>
> Thanks,
> Chang