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

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Message-ID: <BFF7E955-B3BB-45A2-8A01-00ED8971C8D7@intel.com>
Date:   Fri, 15 Jan 2021 04:59:35 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     "Liu, Jing2" <jing2.liu@...ux.intel.com>
CC:     "Liu, Jing2" <jing2.liu@...el.com>, "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 11, 2021, at 18:52, Liu, Jing2 <jing2.liu@...ux.intel.com> wrote:
> 
> 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);

<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. 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