linux-kernel - Re: [PATCH 2/2] selftests/x86/amx: Fix the test to avoid failure when AMX is unavailable

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Date:   Fri, 17 Jun 2022 15:21:18 -0700
From:   "Chang S. Bae" <chang.seok.bae@...el.com>
To:     Shuah Khan <skhan@...uxfoundation.org>,
        <linux-kselftest@...r.kernel.org>, <shuah@...nel.org>,
        <linux-kernel@...r.kernel.org>
CC:     <dave.hansen@...ux.intel.com>, <tglx@...utronix.de>, <bp@...e.de>
Subject: Re: [PATCH 2/2] selftests/x86/amx: Fix the test to avoid failure when
 AMX is unavailable

On 6/16/2022 3:54 PM, Shuah Khan wrote:
> On 4/1/22 4:10 PM, Chang S. Bae wrote:
>>
>> +
>> +static struct {
>> +    unsigned xsave:   1;
>> +    unsigned osxsave: 1;
>> +} cpuinfo;
>> +
> 
> Why is this needed? Also naming this cpuinfo is confuing.

This came from the below CPUID check which seems to be moot.

> 
>>   static inline void check_cpuid_xsave(void)
>>   {
>>       uint32_t eax, ebx, ecx, edx;
>> @@ -118,10 +124,8 @@ static inline void check_cpuid_xsave(void)
>>       eax = 1;
>>       ecx = 0;
>>       cpuid(&eax, &ebx, &ecx, &edx);
>> -    if (!(ecx & CPUID_LEAF1_ECX_XSAVE_MASK))
>> -        fatal_error("cpuid: no CPU xsave support");
>> -    if (!(ecx & CPUID_LEAF1_ECX_OSXSAVE_MASK))
>> -        fatal_error("cpuid: no OS xsave support");
>> +    cpuinfo.xsave = !!(ecx & CPUID_LEAF1_ECX_XSAVE_MASK);
>> +    cpuinfo.osxsave = !!(ecx & CPUID_LEAF1_ECX_OSXSAVE_MASK);
> 
> Why add this complexity. Why not just Skip here?

I think these CPUID checks can go away with ARCH_GET_XCOMP_SUPP.

> 
>>   }
>>   static uint32_t xbuf_size;
>> @@ -161,14 +165,31 @@ static void check_cpuid_xtiledata(void)
>>        * eax: XTILEDATA state component size
>>        * ebx: XTILEDATA state component offset in user buffer
>>        */
>> -    if (!eax || !ebx)
>> -        fatal_error("xstate cpuid: invalid tile data size/offset: 
>> %d/%d",
>> -                eax, ebx);
>> -
>>       xtiledata.size          = eax;
>>       xtiledata.xbuf_offset = ebx;
>>   }
>> +static bool amx_available(void)
>> +{
>> +    check_cpuid_xsave();
>> +    if (!cpuinfo.xsave) {
>> +        printf("[SKIP]\tcpuid: no CPU xsave support\n");
>> +        return false;
>> +    } else if (!cpuinfo.osxsave) {
>> +        printf("[SKIP]\tcpuid: no OS xsave support\n");
>> +        return false;
>> +    }
>> +
>> +    check_cpuid_xtiledata();
>> +    if (!xtiledata.size || !xtiledata.xbuf_offset) {
>> +        printf("[SKIP]\txstate cpuid: no tile data (size/offset: 
>> %d/%d)\n",
>> +               xtiledata.size, xtiledata.xbuf_offset);
>> +        return false;
>> +    }
>> +
>> +    return true;
>> +}
>> +
> 
> I am not seeing any value in adding this layer of abstraction.
> Keep it simple and do the handling in main()

Sure.

> 
>>   /* The helpers for managing XSAVE buffer and tile states: */
>>   struct xsave_buffer *alloc_xbuf(void)
>> @@ -826,9 +847,8 @@ static void test_context_switch(void)
>>   int main(void)
>>   {
>> -    /* Check hardware availability at first */
>> -    check_cpuid_xsave();
>> -    check_cpuid_xtiledata();
>> +    if (!amx_available())
>> +        return 0;
> 
> This should KSFT_SKIP for this to be reported as a skip. Returning 0
> will be reported as a Pass.

I think that's a good point, thanks.

Now, along with the on-going documentation [1], this test code can be 
simplified like the below changes, instead of having those cpuid functions:

diff --git a/tools/testing/selftests/x86/amx.c 
b/tools/testing/selftests/x86/amx.c
index 625e42901237..83705c472a5c 100644
--- a/tools/testing/selftests/x86/amx.c
+++ b/tools/testing/selftests/x86/amx.c
@@ -348,6 +348,7 @@ enum expected_result { FAIL_EXPECTED, 
SUCCESS_EXPECTED };

  /* arch_prctl() and sigaltstack() test */

+#define ARCH_GET_XCOMP_SUPP    0x1021
  #define ARCH_GET_XCOMP_PERM    0x1022
  #define ARCH_REQ_XCOMP_PERM    0x1023

@@ -828,9 +829,14 @@ static void test_context_switch(void)

  int main(void)
  {
-       /* Check hardware availability at first */
-       check_cpuid_xsave();
-       check_cpuid_xtiledata();
+       unsigned long features;
+       long rc;
+
+       rc = syscall(SYS_arch_prctl, ARCH_GET_XCOMP_SUPP, &features);
+       if (rc || (features & XFEATURE_MASK_XTILE) != XFEATURE_MASK_XTILE) {
+               printf("[SKIP]\tno AMX support\n");
+               exit(KSFT_FAIL);
+       }

         init_stashed_xsave();
         sethandler(SIGILL, handle_noperm, 0);

Thanks,
Chang

[1] 
https://lore.kernel.org/lkml/86952726-53e6-17a9-dbe0-3e970c565044@intel.com/