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Date: Tue, 1 Nov 2016 09:46:41 +0100
From: Borislav Petkov <bp@alien8.de>
To: He Chen <he.chen@linux.intel.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
        "Luc, Piotr" <Piotr.Luc@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
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        "Kang, Luwei" <luwei.kang@intel.com>,
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Subject: Re: [PATCH] x86/cpuid: expose AVX512_4VNNIW and AVX512_4FMAPS
 features to kvm guest
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On Tue, Nov 01, 2016 at 03:48:50PM +0800, He Chen wrote:
> Before sending a patch, let me check if my understanding is right...
> I will add a helper in scattered.c like:
> 
> unsigned int get_scattered_cpuid_features(unsigned int level,
> 					unsigned int sub_leaf, enum cpuid_regs reg)
> {
> 	u32 val = 0;
> 	const struct cpuid_bit *cb;
> 
> 	for (cb = cpuid_bits; cb->feature; cb++) {

Right, we can improve that by keeping cpuid_bit.level sorted so that you
can break out early if the level is exceeded. For that please sort it
and add a comment stating that the leaf should be kept sorted ontop of
it.

And then you do something like this:

u32 get_scattered_cpuid_leaf(unsigned int level, unsigned int sub_leaf,
			     enum cpuid_regs reg)
{
	u32 cpuid_val = 0;

	for (cb = cpuid_bits; cb->feature; cb++) {

		if (level < cb->level)
			continue;

		if (level > cb->level)
			break;

		if (cb->reg == reg && sub_leaf == cb->sub_leaf) {
			if (cpu_has(cb->feature))
				cpuid_val |= BIT(cb->bit);
		}
	}

	return cpuid_val;
}

Completely untested, of course.

-- 
Regards/Gruss,
    Boris.

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