linux-kernel - Re: [PATCH v2] KVM: x86: Add Intel CPUID.1F cpuid emulation support

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Date:   Fri, 26 Apr 2019 07:13:46 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Like Xu <like.xu@...ux.intel.com>
Cc:     kvm@...r.kernel.org, Xiaoyao Li <xiaoyao.li@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: Add Intel CPUID.1F cpuid emulation support

On Fri, Apr 26, 2019 at 11:17:52AM +0800, Like Xu wrote:
> Some new systems have multiple software-visible die within each package.
> Add support to expose Intel V2 Extended Topology Enumeration Leaf CPUID.1F.
> 
> Co-developed-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
> Signed-off-by: Xiaoyao Li <xiaoyao.li@...ux.intel.com>
> Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> ---
> 
> ==changelog==
> v2:
> - Apply cpuid.1f check rule on Intel SDM page 3-222 Vol.2A
> - Add comment to handle 0x1f anf 0xb in common code
> - Reduce check time in a descending-break style
> 
> v1: https://lkml.org/lkml/2019/4/22/28
> 
>  arch/x86/kvm/cpuid.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> index fd39516..f9b529e 100644
> --- a/arch/x86/kvm/cpuid.c
> +++ b/arch/x86/kvm/cpuid.c
> @@ -425,6 +425,11 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  
>  	switch (function) {
>  	case 0:
> +		/* Check if the cpuid leaf 0x1f is actually implemented */
> +		if (entry->eax >= 0x1f && (cpuid_ebx(0x1f) & 0x0000ffff)) {

Restricting the check to bits 15:0 is unnecessary, the SDM explicitly
states that EBX will be zero for invalid sub-leaves:

  For sub-leaves that return an invalid level-type of 0 in ECX[15:8];
  EAX and EBX will return 0.

This code is merely checking for the existence of CPUID.1F, nothing will
break if future CPUs provide additional information, i.e. checking for a
valid sub-leaf is sufficient.

That being said, if you insist on restricting the check to non-reserved
bits then I think the earlier suggestion of "cpuid_ecx(0x1f) & 0x0000ff00"
makes more sense since the SDM clearly intends ECX to be used to detect
valid vs. invalid levels.

> +			entry->eax = 0x1f;
> +			break;

I find if/else easier to follow than the separate break, but either option
works for me.
		
> +		}
>  		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
>  		break;
>  	case 1:
> @@ -544,7 +549,12 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>  		entry->edx = edx.full;
>  		break;
>  	}
> -	/* function 0xb has additional index. */
> +	/*
> +	 * Intel documentation states that 0x1f and 0xb have
> +	 * identical formats and thus can be handled by common code.
> +	 * (Intel SDM Vol. 2A - Instruction Set Reference - CPUID)
> +	 */

It's probably safe to assume anyone reading reading this code is already
all too aware of Intel's propensity for dumping CPUID enumeration into the
CPUID entry in the ISR.  Maybe shorten this to something like:

	/*
	 * Per Intel's SDM, 0x1f is a superset of 0xb, thus they can be handled
	 * by common code.
	 */

> +	case 0x1f:
>  	case 0xb: {
>  		int i, level_type;
>  
> -- 
> 1.8.3.1
>