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Date:   Thu, 25 Apr 2019 14:30:09 +0800
From:   Xiaoyao Li <xiaoyao.li@...el.com>
To:     Like Xu <like.xu@...ux.intel.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     kvm@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Len Brown <lenb@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Add Intel CPUID.1F cpuid emulation support

On Thu, 2019-04-25 at 14:02 +0800, Like Xu wrote:
> On 2019/4/25 12:18, Xiaoyao Li wrote:
> > On Thu, 2019-04-25 at 10:58 +0800, Like Xu wrote:
> > > On 2019/4/24 22:32, Sean Christopherson wrote:
> > > > Now that I understand how min() works...
> > > > 
> > > > On Mon, Apr 22, 2019 at 02:40:34PM +0800, Like Xu wrote:
> > > > > Expose Intel V2 Extended Topology Enumeration Leaf to guest only when
> > > > > host system has multiple software-visible die within each package.
> > > > > 
> > > > > Signed-off-by: Like Xu <like.xu@...ux.intel.com>
> > > > > ---
> > > > >    arch/x86/kvm/cpuid.c | 13 +++++++++++++
> > > > >    1 file changed, 13 insertions(+)
> > > > > 
> > > > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > > > index fd39516..9fc14f2 100644
> > > > > --- a/arch/x86/kvm/cpuid.c
> > > > > +++ b/arch/x86/kvm/cpuid.c
> > > > > @@ -65,6 +65,16 @@ u64 kvm_supported_xcr0(void)
> > > > >    	return xcr0;
> > > > >    }
> > > > >    
> > > > > +/* We need to check if the host cpu has multi-chip packaging
> > > > > technology.
> > > > > */
> > > > > +static bool kvm_supported_intel_mcp(void)
> > > > > +{
> > > > > +	u32 eax, ignored;
> > > > > +
> > > > > +	cpuid_count(0x1f, 0, &eax, &ignored, &ignored, &ignored);
> > > > 
> > > > This is broken because of how CPUID works for unsupported input leafs:
> > > > 
> > > >     If a value entered for CPUID.EAX is higher than the maximum input
> > > > value
> > > >     for basic or extended function for that processor then the data for
> > > > the
> > > >     highest basic information leaf is returned.
> > > > 
> > > > For example, my system with a max basic leaf of 0x16 returns 0x00000e74
> > > > for CPUID.1F.EAX.
> > > 
> > > You're right and the cpuid.1f.eax check is unreliable after I checked a
> > > few machines.
> > > 
> > > > 
> > > > > +
> > > > > +	return boot_cpu_data.x86_vendor == X86_VENDOR_INTEL && (eax !=
> > > > > 0);
> > > > 
> > > > Checking 'eax != 0' is broken as it will be '0' when SMT is
> > > > disabled.  ecx
> > > > is the obvious choice since bits 15:8 are guaranteed to be non-zero when
> > > > the leaf is valid.
> > > 
> > > I agree with this and ecx[15:8] makes sense.
> > > 
> > > > 
> > > > I think we can skip the vendor check.  AFAIK, CPUID.1F isn't used by
> > > > AMD,
> > > > and since AMD and Intel try to maintain a semblance of CPUID
> > > > compatibility
> > > > it seems more likely that AMD/Hygon would implement CPUID.1F as-is
> > > > rather
> > > > than repurpose it to mean something else entirely.
> > > 
> > > If it's true, let's skip the vendor check.
> > > 
> > > // I have to mention that AMD already has MCP CPUs.
> > > 
> > > > 
> > > > > +}
> > > > > +
> > > > >    #define F(x) bit(X86_FEATURE_##x)
> > > > >    
> > > > >    int kvm_update_cpuid(struct kvm_vcpu *vcpu)
> > > > > @@ -426,6 +436,7 @@ static inline int __do_cpuid_ent(struct
> > > > > kvm_cpuid_entry2 *entry, u32 function,
> > > > >    	switch (function) {
> > > > >    	case 0:
> > > > >    		entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 :
> > > > > 0xd));
> > > > > +		entry->eax = kvm_supported_intel_mcp() ? 0x1f : entry-
> > > > > >eax;
> > > > 
> > > > If we put everything together, I think the code can be reduced to:
> > > > 
> > > > 		/* comment about multi-chip leaf... */
> > > > 		if (entry->eax >= 0x1f && cpuid_ecx(0x1f))
> > > > 			entry->eax = 0x1f;
> > > > 		else
> > > > 			entry->eax = min(entry->eax,
> > > > 					 (u32)(f_intel_pt ? 0x14 :
> > > > 0xd));
> > > 
> > > Based on:
> > > 
> > > 	ECX Bits 07 - 00: Level number. Same value in ECX input.
> > > 	Bits 15 - 08: Level type.
> > > 	Bits 31 - 16: Reserved.
> > > 
> > > how about using an increasing order:
> > > 
> > > 	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
> > > 
> > > 	// ... more checks when eax is between 0x14 and 0x1f if any
> > > 
> > > 	/* Check if the host cpu has multi-chip packaging technology.*/
> > > 	if (((cpuid_ecx(0x1f) >> 8) & 0xff) != 0)
> > > 		entry->eax = 0x1f;
> > 
> > As Sean pointed out, you cannot rely on the output of cpuid.1f to indicate
> > the
> > existence of leaf 1f. If maximum basic leaf supported is smaller than 1f,
> > the
> > data returned by cpuid_ecx(0x1f) is the actual highest basic information
> > leaf of
> > the hardware.
> 
> I don't think so.
> 
> > So using "entry->eax >= 0x1f" from cpuid.0H is and only is the right way to
> > check the existence of leaf 1f.
> > 
> > We can simply use (cpuid_ecx(0x1f) & 0x0000ff00) to avoid the unnecessory
> > shifting operation.
> 
> I borrowed this "unnecessory" shifting operation from host 
> check_extended_topology_leaf() and we may do better on this.
> 
> > Besides, the problem of simply using cpuid_exc(0x1f) in Sean's codes is that
> > we
> > cannot assmue the reserved bits 31:16 of ECX is always 0 for the future
> > generation.
> 
> It's true cause the statement in public spec is not "Reserved = 0" but 
> "Bits 31 - 16: Reserved".
> 
> > 
> > In my opinion, Sean's codes is OK and much simple and clear.
> 
> If the host cpuid.0.eax is greater than 0x1f but actually it doesn't
> have multi-chip packaging technology and we may want to expose 
> entry->eax to some value smaller than 0x1f but greater than 0x14, much 
> effort needs to apply on Sean's code.
> 
> My improvement is good to overwrite cpuid.0.eax in future usage
> from the perspective of kvm feature setting not just from value check.

Alright, there is something wrong in your code that you haven't realised. 

When you do 
	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));

it changes the entry->eax if entry->eax > 0x14. So you cannot directly use
cpuid_ecx(0x1f). At least, you need to cache the value of entry->eax, like:
	
	u32 max_leaf = entry->eax;
	entry->eax = min(entry->eax, (u32)(f_intel_pt ? 0x14 : 0xd));
	
	//...leaf between 0x14 and 0x1f
	
	if (max_leaf >= 0x1f && (cpuid_ecx(0x1f) & 0x0000ff00))
 		entry->eax = 0x1f;

However, handling in increasing order in totally wrong. Since it's to report the
max the leaf supported, we should handle in descending order, which is what Sean
does.

> > All need to do is using (cpuid_ecx(0x1f) & 0x0000ff00) to verify the leaf.1f
> > is
> > valid.
> > 
> > Thanks,
> > -Xiaoyao
> > > 	// ... more checks when eax greater than 0x1f if any
> > > 
> > > are we OK with it?
> > > 
> > > > >    		break;
> > > > >    	case 1:
> > > > >    		entry->edx &= kvm_cpuid_1_edx_x86_features;
> > > > > @@ -544,6 +555,8 @@ static inline int __do_cpuid_ent(struct
> > > > > kvm_cpuid_entry2 *entry, u32 function,
> > > > >    		entry->edx = edx.full;
> > > > >    		break;
> > > > >    	}
> > > > > +	/* function 0x1f has additional index. */
> > > > > +	case 0x1f:
> > > > >    	/* function 0xb has additional index. */
> > > > >    	case 0xb: {
> > > > >    		int i, level_type;
> > > > > -- 
> > > > > 1.8.3.1
> > > > > 
> > > 
> > > 
> > 
> > 
> 
> 

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