linux-kernel - Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full instruction mask

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Date:   Mon, 22 Jun 2020 10:52:24 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, acme@...nel.org, tglx@...utronix.de,
        bp@...en8.de, x86@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...hat.com, namhyung@...nel.org, dave.hansen@...el.com,
        yu-cheng.yu@...el.com, bigeasy@...utronix.de, gorcunov@...il.com,
        hpa@...or.com, alexey.budankov@...ux.intel.com, eranian@...gle.com,
        ak@...ux.intel.com, like.xu@...ux.intel.com,
        yao.jin@...ux.intel.com
Subject: Re: [PATCH 17/21] x86/fpu: Use proper mask to replace full
 instruction mask



On 6/19/2020 3:31 PM, Peter Zijlstra wrote:
> On Fri, Jun 19, 2020 at 07:04:05AM -0700, kan.liang@...ux.intel.com wrote:
> 
>> KVM includes the header file fpu/internal.h. To avoid 'undefined
>> xfeatures_mask_all' compiling issue, xfeatures_mask_all has to be
>> exported.
> 
>> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
>> index 587e03f..eb2e44e 100644
>> --- a/arch/x86/kernel/fpu/xstate.c
>> +++ b/arch/x86/kernel/fpu/xstate.c
>> @@ -58,6 +58,7 @@ static short xsave_cpuid_features[] __initdata = {
>>    * XSAVE buffer, both supervisor and user xstates.
>>    */
>>   u64 xfeatures_mask_all __read_mostly;
>> +EXPORT_SYMBOL_GPL(xfeatures_mask_all);
> 
> *groan*...
> 
> AFAICT KVM doesn't actually use any of those functions,

It seems KVM may eventually invoke copy_xregs_to_kernel() as below.

kvm_save_current_fpu()
     copy_fpregs_to_fpstate()
         copy_xregs_to_kernel()

I think we have to export the xfeatures_mask_all.

Thanks,
Kan

> can't we have
> something like BUILD_KVM (like BUILD_VDSO) and exclude those functions
> from the KVM build?
> 
> I so detest exporting random crap because kvm..
>