linux-kernel - Re: [PATCH] x86/fpu: Warn only when CPU-provided sizes less than struct declaration

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Message-ID: <68bdd6f0-a229-433a-9234-303a3b02b092@amd.com>
Date:   Fri, 6 Dec 2019 15:14:13 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        x86@...nel.org
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        jon.grimm@....com,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Thomas Lendacky <Thomas.Lendacky@....com>
Subject: Re: [PATCH] x86/fpu: Warn only when CPU-provided sizes less than
 struct declaration

Dave,

On 12/4/19 12:27 AM, Dave Hansen wrote:
> On 12/3/19 1:01 AM, Suravee Suthikulpanit wrote:
>> The current XCHECK_SZ macro warns if the XFEATURE size reported
>> by CPUID does not match the size of kernel structure. However, depending
>> on the hardware implementation, CPUID can report the XSAVE state size
>> larger than the size of C structures defined for each of the XSAVE state
>> due to padding.
> 
> We have existing architecture for padding.  See xfeature_is_aligned(),
> for instance.  Are you saying that there are implementations out there
> that do padding which is not otherwise enumerated and that they do it
> within the size of the enumerated stat
Yes, the implementation includes the padding size within the size of
the enumerated state. This results in the reported size larger than
the amount needed by the feature.

>> Such case should be safe and should not need to generate warning
>> message.
> 
> I've seen these error messages trip before, but only on pre-production
> processors with goofy microcode.  I'd be really suspicious that this is
> just papering over a processor issue.  Or, that perhaps the compacted
> form works but the standard form is broken somehow.

I have verified with the HW folks and the have confirmed that this is
to be expected.

Thanks,
Suravee