linux-kernel - Re: [PATCH] x86/cpuid: Fewer CPUID invocations in init_scattered_cpuid

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Date:   Tue, 23 Jan 2018 18:32:16 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     David Woodhouse <dwmw@...zon.co.uk>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86/cpuid: Fewer CPUID invocations in
 init_scattered_cpuid_features()

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@...zon.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.