linux-kernel - Re: [PATCH v3 05/21] x86/fpu/xstate: Add a new variable to indicate dynamic user states

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Date:   Fri, 15 Jan 2021 19:47:38 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Borislav Petkov <bp@...e.de>
CC:     Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, x86-ml <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/21] x86/fpu/xstate: Add a new variable to indicate
 dynamic user states

On Jan 15, 2021, at 05:39, Borislav Petkov <bp@...e.de> wrote:
> On Wed, Dec 23, 2020 at 07:57:01AM -0800, Chang S. Bae wrote:
>> The perf has a buffer that is allocated on demand. The states saved in the
> 
> What's "the perf"? I hope to find out when I countinue reading…

Maybe it was better to write ‘Linux perf (tools)’ [1] here. Sorry for the
confusion.

>> diff --git a/arch/x86/include/asm/fpu/xstate.h b/arch/x86/include/asm/fpu/xstate.h
>> index 24bf8d3f559a..6ce8350672c2 100644
>> --- a/arch/x86/include/asm/fpu/xstate.h
>> +++ b/arch/x86/include/asm/fpu/xstate.h
>> @@ -56,7 +56,7 @@
>>  * - Don't set the bit corresponding to the dynamic supervisor feature in
>>  *   IA32_XSS at run time, since it has been set at boot time.
>>  */
>> -#define XFEATURE_MASK_DYNAMIC (XFEATURE_MASK_LBR)
>> +#define XFEATURE_MASK_SUPERVISOR_DYNAMIC (XFEATURE_MASK_LBR)
> 
> Is XFEATURE_MASK_USER_DYNAMIC coming too?

Instead of the new define, I thought the new variable --
xfeatures_mask_user_dynamic, as its value needs to be determined at boot
time.

PATCH13/21 has the routine:

        xfeatures_mask_all &= fpu__get_supported_xfeatures_mask();
-       /* Do not support the dynamically allocated buffer yet. */
        xfeatures_mask_user_dynamic = 0;
 
+       for (i = FIRST_EXTENDED_XFEATURE; i < XFEATURE_MAX; i++) {
+               u64 feature_mask = BIT_ULL(i);
+
+               if (!(xfeatures_mask_user() & feature_mask))
+                       continue;
+
+               if (xfeature_disable_supported(i))
+                       xfeatures_mask_user_dynamic |= feature_mask;
+       }
+

Thanks,
Chang

[1] https://en.wikipedia.org/wiki/Perf_(Linux)