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

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Date:   Thu, 7 Jan 2021 18:40:21 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Liu, Jing2" <jing2.liu@...el.com>
CC:     "bp@...e.de" <bp@...e.de>, "luto@...nel.org" <luto@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>
Subject: Re: [PATCH v3 10/21] x86/fpu/xstate: Update xstate save function to
 support dynamic xstate


> On Jan 7, 2021, at 17:41, Liu, Jing2 <jing2.liu@...el.com> 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);
> For kvm, if we assume that it does not support dynamic features until this series,
> memcpy for only fpu->state is correct. 
> I think this kind of assumption is reasonable and we only make original xstate work.
> 
> -	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?

Thanks,
Chang