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Subject: Re: x86/fpu/xsave: protection key test failures
To:     Dave Hansen <dave.hansen@intel.com>,
        Babu Moger <babu.moger@amd.com>, linux-kernel@vger.kernel.org,
        linux-kselftest@vger.kernel.org
Cc:     tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, x86@kernel.org,
        hpa@zytor.com, dave.hansen@linux.intel.com, luto@kernel.org,
        peterz@infradead.org, shuah@kernel.org, jroedel@suse.de,
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        fenghua.yu@intel.com, kan.liang@linux.intel.com,
        akpm@linux-foundation.org, rppt@kernel.org, Fan_Yang@sjtu.edu.cn,
        anshuman.khandual@arm.com, b.thiel@posteo.de, jgross@suse.com,
        keescook@chromium.org, seanjc@google.com, mh@glandium.org,
        sashal@kernel.org, krisman@collabora.com, chang.seok.bae@intel.com,
        0x7f454c46@gmail.com, jhubbard@nvidia.com, sandipan@linux.ibm.com,
        ziy@nvidia.com, kirill.shutemov@linux.intel.com,
        suxingxing@loongson.cn, harish@linux.ibm.com,
        rong.a.chen@intel.com, linuxram@us.ibm.com, bauerman@linux.ibm.com
References: <b2e0324a-9125-bb34-9e76-81817df27c48@amd.com>
 <7a407363-e074-aa84-3ca1-909b497122aa@intel.com>
From:   Dave Kleikamp <dave.kleikamp@oracle.com>
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On 5/25/21 5:18 PM, Dave Hansen wrote:
> On 5/25/21 2:37 PM, Babu Moger wrote:
>> My suspicion at this point is towards the selftest tool protection_keys.c.
>> I will keep looking. Any feedback would be much appreciated to debug further.
> 
> The pkey selftest code that pokes at the signal stack is rather hackish.
>   If I had to guess, I'd suspect that PKRU ends up in there in a slightly
> different place than on Intel CPUs.
> 
> One oddity is that xfeatures seems to lose its pkey bit somewhere:
> 
>>   protection_keys-17350 [035] 59275.834197: x86_fpu_copy_src:     	x86/fpu: 0xffff93d7595e2dc0 load: 0 xfeatures: 202 xcomp_bv: 8000000000000207
>>   protection_keys-17350 [035] 59275.834197: x86_fpu_copy_dst:     	x86/fpu: 0xffff93d722877800 load: 0 xfeatures: 2 xcomp_bv: 8000000000000207
> 
> The only legitimate way that can happen (on Intel at least) is an XRSTOR
> that brings PKRU back to the init state.  That would destroy all
> meaningful PKRU state, unless PKRU=0, which it almost never does on Linux.
> 
> What values do PKRU and the shadow have when the test fails?  Is PKRU 0?
>   Any idea how xfeatures&0x200 got clear?

I did observe that PKRU is 0 right before the failure.

Shouldn't this still be a valid value?