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 13:47:22 -0400
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Dave Hansen <dave.hansen@...el.com>,
        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, 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 6/22/2020 11:02 AM, Dave Hansen wrote:
> On 6/22/20 7:52 AM, Liang, Kan wrote:
>>>> --- 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.
> 
> I'm wondering if we should just take these copy_*regs_to_*() functions
> and uninline them.  Yeah, they are basically wrapping one instruction,
> but it might literally be the most heavyweight instruction in the whole ISA.
>

Thanks for the suggestions, but I'm not sure if I follow these methods.

I don't think simply removing the "inline" key word for the 
copy_xregs_to_kernel() functions would help here.
Do you mean exporting the copy_*regs_to_*()?


> Or, maybe just make an out-of-line version for KVM to call?
> 

I think the out-of-line version for KVM still needs the 
xfeatures_mask_all. Because the size of vcpu's XSAVE buffer 
(&vcpu->arch.guest_fpu) is the same as other kernel XSAVE buffers, such 
as task->fpu. The xfeatures_mask_all is required for KVM to filter out 
the dynamic supervisor feature as well. I think even if we make an 
out-of-line version for KVM, we still have to export the 
xfeatures_mask_all for KVM.


Thanks,
Kan