Messages in this thread | | | Date | Thu, 17 Nov 2011 13:09:56 +0000 | From | "Jan Beulich" <> | Subject | Re: your patch "crypto: sha1 - SSSE3 based SHA1 implementation for x86-64" vs xsave |
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>>> On 17.11.11 at 13:36, Mathias Krause <minipli@googlemail.com> wrote: > On Thu, Nov 17, 2011 at 11:33 AM, Jan Beulich <JBeulich@suse.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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