Messages in this thread | | | Date | Tue, 23 Jan 2018 18:32:16 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH] x86/cpuid: Fewer CPUID invocations in init_scattered_cpuid_features() |
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On Tue, Jan 23, 2018 at 02:53:33PM +0000, David Woodhouse wrote: > We were doing a fresh CPUID for every single bit in every single output > register. Do it once and then harvest *all* the bits we want. > > We were also doing the max_level check with a new CPUID invocation for > every single bit. Stop that too. > > Signed-off-by: David Woodhouse <dwmw@amazon.co.uk> > --- > Spotted this in my travels; it offended me.
Ok, I see that it's itching so let's scratch it properly :-)
If we're going to optimize scattered.c, let's do something like this:
* do CPUID for each function once. * for each set bit in there, set feature flag
which means we'd have to change the data structure.
struct cpuid_leaf { u32 level; u32 sub_leaf; struct cpuid_bit bits[]; };
and that last thing is:
struct cpuid_bit { u16 feature; u8 reg; u8 bit; };
So that you have something like (for example with leaf 0x10):
struct cpuid_leaf leafs[] = { ... { .level = 0x00000010, .sub_leaf = 0, .bits = { { X86_FEATURE_CAT_L3, CPUID_EBX, 1 }, { X86_FEATURE_CAT_L2, CPUID_EBX, 2 }, { X86_FEATURE_MBA , CPUID_EBX, 3 }, { 0 } } } ... }
This way you get the CPUID only once and then iterate over bits[] and you can do the cleaner max level computation
cpuid_eax(level & 0xffff0000);
without having to do the extended level checks.
Anyway, something like that.
It's probably not even worth doing anything though - I doubt the speedup is visible at all.
But I certainly understand the intent to fix an annoying thing like that. :-))
Thx.
-- Regards/Gruss, Boris.
Good mailing practices for 400: avoid top-posting and trim the reply.
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