Re: [PATCH 1/6] KVM: x86: Fix tracing of CPUID.function when function is out-of-range

[Xiaoyao Li]

On 3/3/2020 12:08 PM, Sean Christopherson wrote:
> On Tue, Mar 03, 2020 at 10:50:03AM +0800, Xiaoyao Li wrote:
> > On 3/3/2020 3:57 AM, Sean Christopherson wrote:
> > > Rework kvm_cpuid() to query entry->function when adjusting the output
> > > values so that the original function (in the aptly named "function") is
> > > preserved for tracing.  This fixes a bug where trace_kvm_cpuid() will
> > > trace the max function for a range instead of the requested function if
> > > the requested function is out-of-range and an entry for the max function
> > > exists.
> > >
> > > Fixes: 43561123ab37 ("kvm: x86: Improve emulation of CPUID leaves 0BH and 1FH")
> > > Reported-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
> > > Cc: Jim Mattson <jmattson@xxxxxxxxxx>
> > > Cc: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > > ---
> > >   arch/x86/kvm/cpuid.c | 15 +++++++--------
> > >   1 file changed, 7 insertions(+), 8 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index b1c469446b07..6be012937eba 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -997,12 +997,12 @@ static bool cpuid_function_in_range(struct kvm_vcpu *vcpu, u32 function)
> > >   	return max && function <= max->eax;
> > >   }
> > > +/* Returns true if the requested leaf/function exists in guest CPUID. */
> > >   bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> > >   	       u32 *ecx, u32 *edx, bool check_limit)
> > >   {
> > > -	u32 function = *eax, index = *ecx;
> > > +	const u32 function = *eax, index = *ecx;
> > >   	struct kvm_cpuid_entry2 *entry;
> > > -	struct kvm_cpuid_entry2 *max;
> > >   	bool found;
> > >   	entry = kvm_find_cpuid_entry(vcpu, function, index);
> > > @@ -1015,18 +1015,17 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
> > >   	 */
> > >   	if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
> > >   	    !cpuid_function_in_range(vcpu, function)) {
> > > -		max = kvm_find_cpuid_entry(vcpu, 0, 0);
> > > -		if (max) {
> > > -			function = max->eax;
> > > -			entry = kvm_find_cpuid_entry(vcpu, function, index);
> > > -		}
> > > +		entry = kvm_find_cpuid_entry(vcpu, 0, 0);
> > > +		if (entry)
> > > +			entry = kvm_find_cpuid_entry(vcpu, entry->eax, index);
> >
> > There is a problem.
> >
> > when queried leaf is out of range on Intel CPU, it returns the maximum basic
> > leaf, and any dependence on input ECX (i.e., subleaf) value in the basic
> > leaf is honored. As disclaimed in SDM of CPUID instruction.
>
> That's what the code above does.
>
> > The ECX should be honored if and only the leaf has a significant index.
> > If the leaf doesn't has a significant index, it just ignores the EDX input
>
> s/EDX/ECX
>
> > in bare metal.
> >
> > So it should be something like:
> >
> > if (!entry && check_limit && !guest_cpuid_is_amd(vcpu) &&
> > 	!cpuid_function_in_range(vcpu, function)) {
> > 	entry = kvm_find_cpuid_entry(vcpu, 0, 0);
> > 	if (entry) {
> > 		entry = kvm_find_cpuid_entry(vcpu, entry->eax, 0);
> > 		if (entry &&
> > 		    entry->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX ) {
>
> This is unnecessary IMO.  The only scenario where SIGNFICANT_INDEX is 0
> and cpuid_entry(entry->eax, 0) != cpuid_entry(entry->eax, index) is if
> userspace created a cpuid entry for index>0 with SIGNFICANT_INDEX.  

I just forgot that is_matching_cpuid_entry() has taken SIGNIFICANT_INDEX 
into account.

Please ignore my stupid noise.

> a busted model, e.g. it'd be the SDM equivalent of an Intel CPU having
> different output for CPUID.0x16.0 and CPUID.16.5 despite the SDM stating
> that the CPUID.0x16 ignores the index.
>
> E.g. on my system with a max basic leaf of 0x16
>
> $ cpuid -1 -r
> CPU:
>     0x00000000 0x00: eax=0x00000016 ebx=0x756e6547 ecx=0x6c65746e edx=0x49656e69
> ...
>     0x00000016 0x00: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
>
> $ cpuid -1 -r -l 0x16
> CPU:
>     0x00000016 0x00: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> ~ $ cpuid -1 -r -l 0x16 -s 4
> CPU:
>     0x00000016 0x04: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
> ~ $ cpuid -1 -r -l 0x16 -s 466
> CPU:
>     0x00000016 0x1d2: eax=0x00000e74 ebx=0x0000125c ecx=0x00000064 edx=0x00000000
>
>
> If it returned anything else for CPUID.0x16.0x4 then it'd be a CPU bug.
> Same thing here, it's a userspace bug if it creates a CPUID entry that
> shouldn't exist.  E.g. ignoring Intel's silly "max basic leaf" behavior
> for the moment, if userspace created a entry for CPUID.0x0.N it would
> break the Linux kernel's cpu_detect(), as it doesn't initialize ECX when
> doing CPUID.0x0.
>
> > 			entry = kvm_find_cpuid_entry(vcpu, entry->eax,
> > 						     index);
> > 		}
> > 	}
> > }
> >
> > >   	}
> > >   	if (entry) {
> > >   		*eax = entry->eax;
> > >   		*ebx = entry->ebx;
> > >   		*ecx = entry->ecx;
> > >   		*edx = entry->edx;
> > > -		if (function == 7 && index == 0) {
> > > +
> > > +		if (entry->function == 7 && index == 0) {
> > >   			u64 data;
> > >   		        if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
> > >   			    (data & TSX_CTRL_CPUID_CLEAR))