linux-kernel - Re: [PATCH] x86/fpu/xstate: Fix an xstate size check warning

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Message-ID: <7fbe722b-7a75-098d-23b5-fd1fd7d310d6@linux.intel.com>
Date:   Tue, 21 Jul 2020 14:27:17 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Cyrill Gorcunov <gorcunov@...il.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        tglx@...utronix.de, bp@...en8.de, x86@...nel.org,
        linux-kernel@...r.kernel.org, dave.hansen@...el.com,
        yu-cheng.yu@...el.com, bigeasy@...utronix.de, hpa@...or.com,
        eranian@...gle.com, ak@...ux.intel.com, chang.seok.bae@...el.com
Subject: Re: [PATCH] x86/fpu/xstate: Fix an xstate size check warning



On 7/20/2020 1:33 PM, Cyrill Gorcunov wrote:
> On Mon, Jul 20, 2020 at 06:50:51AM -0700, kan.liang@...ux.intel.com wrote:
> ...
>>   static unsigned int __init get_xsave_size(void)
>>   {
>>   	unsigned int eax, ebx, ecx, edx;
>> @@ -710,7 +741,7 @@ static int __init init_xstate_size(void)
>>   	xsave_size = get_xsave_size();
>>   
>>   	if (boot_cpu_has(X86_FEATURE_XSAVES))
>> -		possible_xstate_size = get_xsaves_size();
>> +		possible_xstate_size = get_xsaves_size_no_dynamic();
>>   	else
>>   		possible_xstate_size = xsave_size;
> 
> Hi! Maybe we could enhance get_xsaves_size instead ? The get_xsaves_size is
> static and __init function (thus not a hot path) used once as far as I see.
> Say
> 
> static unsigned int __init get_xsaves_size(void)
> {
> 	u64 mask = xfeatures_mask_dynamic();
> 	unsigned int eax, ebx, ecx, edx;
> 
> 	/*
> 	 * In case if dynamic features are present make
> 	 * sure they are not accounted in the result since
> 	 * the buffer should be allocated separately from
> 	 * task->fpu.
> 	 */
> 	if (mask)
> 		wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor());
> 
> 	/*
> 	 * - CPUID function 0DH, sub-function 1:
> 	 *    EBX enumerates the size (in bytes) required by
> 	 *    the XSAVES instruction for an XSAVE area
> 	 *    containing all the state components
> 	 *    corresponding to bits currently set in
> 	 *    XCR0 | IA32_XSS.
> 	 */
> 	cpuid_count(XSTATE_CPUID, 1, &eax, &ebx, &ecx, &edx);
> 
> 	if (mask)
> 		wrmsrl(MSR_IA32_XSS, xfeatures_mask_supervisor() | mask);
> 
> 	return ebx;
> }
> 
> but if you expect more use of get_xsaves_size_no_dynamic() and
> get_xsaves_size() in future then sure, we need a separate function.
> 

For now, I don't have more use of 
get_xsaves_size_no_dynamic()/get_xsaves_size(). I don't know if anyone 
else will use them later.

> The benefit from such extension is that when you read get_xsaves_size
> you'll notice the dependency on dynamic features immediaely.
> 
> Though I'm fine with current patch as well, up to you. Thanks for the patch!
> 

Personally, I prefer to keep the current patch because I like the name 
get_xsaves_size_no_dynamic(), which explicitly tells the dynamic 
features are excluded.

> Reviewed-by: Cyrill Gorcunov <gorcunov@...il.com>
> 

Thanks for the review.

Kan