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

[Liang, Kan]

On 7/20/2020 1:33 PM, Cyrill Gorcunov wrote:
> On Mon, Jul 20, 2020 at 06:50:51AM -0700, kan.liang@xxxxxxxxxxxxxxx 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@xxxxxxxxx>

Thanks for the review.

Kan