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Date: Tue, 23 Jul 2013 13:16:04 +0200
From: Paolo Bonzini <pbonzini@redhat.com>
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To: "H. Peter Anvin" <hpa@zytor.com>
CC: Jason Wang <jasowang@redhat.com>, tglx@linutronix.de, mingo@redhat.com,
        x86@kernel.org, gleb@redhat.com, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 1/4] x86: introduce hypervisor_cpuid_base()
References: <1374572465-15278-1-git-send-email-jasowang@redhat.com> <51EE553C.40305@zytor.com>
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Il 23/07/2013 12:04, H. Peter Anvin ha scritto:
> On 07/23/2013 02:41 AM, Jason Wang wrote:
>>  
>> +static inline uint32_t hypervisor_cpuid_base(const char *sig, uint32_t leaves)
>> +{
>> +	uint32_t base, eax, ebx, ecx, edx;
>> +	char signature[13];
>> +
>> +	for (base = 0x40000000; base < 0x40010000; base += 0x100) {
>> +		cpuid(base, &eax, &ebx, &ecx, &edx);
>> +		*(uint32_t *)(signature + 0) = ebx;
>> +		*(uint32_t *)(signature + 4) = ecx;
>> +		*(uint32_t *)(signature + 8) = edx;
>> +		signature[12] = 0;
>> +
>> +		if (!strcmp(sig, signature) &&
>> +		    (leaves == 0 || ((eax - base) >= leaves)))
>> +			return base;
>> +	}
>> +
>> +	return 0;
>> +}
>> +
> 
> Hmm... how about:
> 
> uint32_t sign[3];
> 
> 	cpuid(base, &eax, &sign[0], &sign[1], &sign[2]);
> 
> 	if (!memcmp(sig, sign, 12) && ...);

That's nicer, though strcmp is what the replaced code used to do in
patches 2 and 3.

Note that memcmp requires the caller to use "KVMKVMKVM\0\0" as the
signature (or alternatively hypervisor_cpuid_base can copy the argument
into another 12-byte local variable).

Paolo