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

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Date:   Tue, 12 Jan 2021 10:52:29 +0800
From:   "Liu, Jing2" <jing2.liu@...ux.intel.com>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "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 1/8/2021 2:40 AM, Bae, Chang Seok wrote:
>> 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?

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).

Thanks,

Jing

> Thanks,
> Chang