linux-kernel - Re: your patch "crypto: sha1 - SSSE3 based SHA1 implementation for x86-64" vs xsave

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Date:	Thu, 17 Nov 2011 13:09:56 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Mathias Krause" <minipli@...glemail.com>
Cc:	<herbert@...dor.apana.org.au>, <linux-kernel@...r.kernel.org>
Subject: Re: your patch "crypto: sha1 - SSSE3 based SHA1 implementation
 for x86-64" vs xsave

>>> On 17.11.11 at 13:36, Mathias Krause <minipli@...glemail.com> wrote:
> On Thu, Nov 17, 2011 at 11:33 AM, Jan Beulich <JBeulich@...e.com> wrote:
>> Mathias,
>>
>> in avx_usable() you use cpu_has_osxsave to determine whether AVX can
>> actually be used, but I can't see how that conditional would ever evaluate
>> to true.
> 
> Well, it does, at least in my setup, a Core i7-2620M:
> 
>   [   44.902613] sha1_ssse3: Using AVX optimized SHA-1 implementation
> 
>> Checking CPUID.OSXSAVE is actually a user land requirement,
>> whereas in the kernel you should really look at kernel internal variables
>> to determine whether xsave was enabled
> 
> That's a valid point. I actually ported the check from the initial
> assembler implementation which was written with userland code in mind.
> Albeit this test can also be used in kernel code because the kernel
> only sets this flag after xsave was enabled in
> arch/x86/kernel/xsave.c:xstate_enable_boot_cpu/xstate_enable

Indeed, I got this the wrong way round.

>> (or, if you really want to stay
>> with using cpu_has_osxsave you'd have to force re-execution of
>> get_cpu_cap() after xsave got enabled on individual CPUs), namely
>> mmu_cr4_features having X86_CR4_OSXSAVE set.
> 
> I don't see where X86_CR4_OSXSAVE would get reset after xsave init.
> Can you give me a pointer to that code?

I didn't say it would get cleared. I said it should be used in favor of
cpu_has_osxsave.

>> Additionally, under a hypervisor, CPUID.OSXSAVE may be set (due to
>> the hypervisor having enabled xsave), while the kernel may be running
>> with xsave disabled (e.g. due to a command line option saying so).
> 
> When noxsave is given on the kernel command line, X86_FEATURE_XSAVE
> will be cleared and xsave_init() won't call xstate_enable(), so not
> setting X86_CR4_OSXSAVE. All fine.

But you're not looking at this bit, you're looking at cpu_has_osxsave,
which is a CPUID bit (not a CR4 one).

Jan

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