Re: [PATCH 1/1] arch/x86/kvm/cpuid.c: cpuid_maxphyaddr "bad"handling.

[Gleb Natapov]

On Fri, Nov 30, 2012 at 03:20:27PM -0200, Raphael S Carvalho wrote:
> Well, the below function reports the physical-address width supported
> by the processor.
> It does its work very well, though I found a detail which it doesn't
> handle at all.
> 
> PS: The following function is not a patch.
> int cpuid_maxphyaddr(struct kvm_vcpu *vcpu)
> {
>         struct kvm_cpuid_entry2 *best;
> 
>         best = kvm_find_cpuid_entry(vcpu, 0x80000000, 0);
>         if (!best || best->eax < 0x80000008)
>                 goto not_found;
>         best = kvm_find_cpuid_entry(vcpu, 0x80000008, 0);
>         if (best)
>                 return best->eax & 0xff;
> not_found:
>         return 36;
> }
> 
> As I'm seeing, its(above function) first step is to check whether the
> CPU provides the CPUID function 80000008H,
> if so, it gets the physical-address width from the available CPUID
> function, otherwise it implicitly returns 36.
> 
> Intel manual says the following:
> "For processors that do not support CPUID function 80000008H, the
> width is generally 36 if CPUID.01H:EDX.PAE [bit 6] = 1
> and 32 otherwise."
> According to the above-mentioned statement, we would have to return 32
> whether PAE is not supported by the CPU.
> So I was wondering if such a function would work efficiently on
> processors that do not support PAE extension.
> I also would like to share that MAXPHYADDR can be at most 52. However,
> not sure if such an implementation is even needed.
> 
> NOTE:
> arch/x86/include/asm/cpufeature.h provides the below macro.
> #define cpu_has_pae             boot_cpu_has(X86_FEATURE_PAE)
> Does the above macro return 1 whether CPU does support the PAE feature?
> If so, we would have to make a single change in the code:
> 
> Signed-off-by: Raphael S.Carvalho <raphael.scarv@xxxxxxxxx>
> 
> --- a/arch/x86/kvm/cpuid.c	2012-11-19 23:44:29.000000000 -0200
> +++ b/arch/x86/kvm/cpuid.c	2012-11-30 15:05:51.000000000 -0200
> @@ -617,7 +617,10 @@
>  	if (best)
>  		return best->eax & 0xff;
>  not_found:
> -	return 36;
> +	/* Check whether CPU supports PAE, if so the MAXPHYADDR
> +	 * is 36, otherwise 32.
> +	 */
> +	return (cpu_has_pae) ? 36 : 32;
>  }
> 
> Follows a different patch otherwise:
> arch/x86/include/asm/processor.h does provide a generic cpuid function
> called native_cpuid, however, I added another procedure for
> simplicity/efficiency purposes.
> 
> Signed-off-by: Raphael S.Carvalho <raphael.scarv@xxxxxxxxx>
> 
> --- a/arch/x86/kvm/cpuid.c	2012-11-19 23:44:29.000000000 -0200
> +++ b/arch/x86/kvm/cpuid.c	2012-11-30 14:28:22.000000000 -0200
> @@ -606,6 +606,30 @@
> 
> +#ifndef PAE_BIT
> +#define PAE_BIT (1ULL << 6)
> +#endif
> +static inline unsigned cpuid_cpu_pae_support(void)
> +{
> +	unsigned int __edx;
> +	const unsigned int cpu_id_param = 0x01;
> +
> +	/* According to Intel Manual we can check
> +	 * whether the processor does provide PAE by
> +	 * using the CPUID instruction.
> +	 * Syntax: CPUID.01H:EDX.PAE [bit 6] = 1
> +	 */
> +	__edx = 0;
> +	asm volatile(
> +		"cpuid"
> +		: "=d"(__edx)
> +		: "a"(cpu_id_param)
> +		: "ecx","ebx"
> +		);
> +
You need to check cpuid as seen by a guest not a host. Something
like this:

struct kvm_cpuid_entry2 *feat = kvm_find_cpuid_entry(vcpu, 1, 0)
if (feat && (feat->edx & PAE_BIT))
    return 36;
else
    return 32;

But currently KVM code assumes that PAE is always present. kvm_set_cr4()
allows PAE to be set without doing the check above.

> +	return (__edx & PAE_BIT);
> +}
> +
>  int cpuid_maxphyaddr(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm_cpuid_entry2 *best;
> @@ -617,7 +641,10 @@
>  	if (best)
>  		return best->eax & 0xff;
>  not_found:
> -	return 36;
> +	/* Check whether CPU supports PAE, if so the MAXPHYADDR
> +	 * is 36, otherwise 32.
> +	 */
> +	return (cpuid_cpu_pae_support()) ? 36 : 32;
>  }
> 
> Regards,
> Raphael S.Carvalho <raphael.scarv@xxxxxxxxx>
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--
			Gleb.
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