Re: [PATCH 1/3] KVM: x86: disconnect kvm_check_cpuid() from vcpu->arch.cpuid_entries

[Maxim Levitsky]

On Mon, 2020-10-05 at 13:51 +0200, Vitaly Kuznetsov wrote:
> Maxim Levitsky <mlevitsk@xxxxxxxxxx> writes:
> 
> > On Thu, 2020-10-01 at 15:05 +0200, Vitaly Kuznetsov wrote:
> > > As a preparatory step to allocating vcpu->arch.cpuid_entries dynamically
> > > make kvm_check_cpuid() check work with an arbitrary 'struct kvm_cpuid_entry2'
> > > array.
> > > 
> > > Currently, when kvm_check_cpuid() fails we reset vcpu->arch.cpuid_nent to
> > > 0 and this is kind of weird, i.e. one would expect CPUIDs to remain
> > > unchanged when KVM_SET_CPUID[2] call fails.
> > Since this specific patch doesn't fix this, maybe move this chunk to following patches,
> > or to the cover letter?
> 
> Basically, this kind of pairs with what's after 'No functional change
> intended' below: we admit the problem but don't fix it because we can't
> (yet) and then in PATCH3 we do two things at once. It would be great to
> separate these two changes but this doesn't seem to be possible without
> an unneeded code churn.
> 
> That said, I'm completely fine with dropping this chunk from the commit
> message if it sound inapropriate here.
It just threw me a bit off course while trying to understand what the patch does.

Best regards,
	Maxim Levitsky

> 
> > > No functional change intended. It would've been possible to move the updated
> > > kvm_check_cpuid() in kvm_vcpu_ioctl_set_cpuid2() and check the supplied
> > > input before we start updating vcpu->arch.cpuid_entries/nent but we
> > > can't do the same in kvm_vcpu_ioctl_set_cpuid() as we'll have to copy
> > > 'struct kvm_cpuid_entry' entries first. The change will be made when
> > > vcpu->arch.cpuid_entries[] array becomes allocated dynamically.
> > > 
> > > Suggested-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> > > Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
> > > ---
> > >  arch/x86/kvm/cpuid.c | 38 +++++++++++++++++++++++---------------
> > >  1 file changed, 23 insertions(+), 15 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > > index 37c3668a774f..529348ddedc1 100644
> > > --- a/arch/x86/kvm/cpuid.c
> > > +++ b/arch/x86/kvm/cpuid.c
> > > @@ -54,7 +54,24 @@ static u32 xstate_required_size(u64 xstate_bv, bool compacted)
> > >  
> > >  #define F feature_bit
> > >  
> > > -static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
> > > +static inline struct kvm_cpuid_entry2 *cpuid_entry2_find(
> > > +	struct kvm_cpuid_entry2 *entries, int nent, u32 function, u32 index)
> > > +{
> > > +	struct kvm_cpuid_entry2 *e;
> > > +	int i;
> > > +
> > > +	for (i = 0; i < nent; i++) {
> > > +		e = &entries[i];
> > > +
> > > +		if (e->function == function && (e->index == index ||
> > > +		    !(e->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX)))
> > > +			return e;
> > > +	}
> > > +
> > > +	return NULL;
> > > +}
> > > +
> > > +static int kvm_check_cpuid(struct kvm_cpuid_entry2 *entries, int nent)
> > >  {
> > >  	struct kvm_cpuid_entry2 *best;
> > >  
> > > @@ -62,7 +79,7 @@ static int kvm_check_cpuid(struct kvm_vcpu *vcpu)
> > >  	 * The existing code assumes virtual address is 48-bit or 57-bit in the
> > >  	 * canonical address checks; exit if it is ever changed.
> > >  	 */
> > > -	best = kvm_find_cpuid_entry(vcpu, 0x80000008, 0);
> > > +	best = cpuid_entry2_find(entries, nent, 0x80000008, 0);
> > >  	if (best) {
> > >  		int vaddr_bits = (best->eax & 0xff00) >> 8;
> > >  
> > > @@ -220,7 +237,7 @@ int kvm_vcpu_ioctl_set_cpuid(struct kvm_vcpu *vcpu,
> > >  		vcpu->arch.cpuid_entries[i].padding[2] = 0;
> > >  	}
> > >  	vcpu->arch.cpuid_nent = cpuid->nent;
> > > -	r = kvm_check_cpuid(vcpu);
> > > +	r = kvm_check_cpuid(vcpu->arch.cpuid_entries, cpuid->nent);
> > >  	if (r) {
> > >  		vcpu->arch.cpuid_nent = 0;
> > >  		kvfree(cpuid_entries);
> > > @@ -250,7 +267,7 @@ int kvm_vcpu_ioctl_set_cpuid2(struct kvm_vcpu *vcpu,
> > >  			   cpuid->nent * sizeof(struct kvm_cpuid_entry2)))
> > >  		goto out;
> > >  	vcpu->arch.cpuid_nent = cpuid->nent;
> > > -	r = kvm_check_cpuid(vcpu);
> > > +	r = kvm_check_cpuid(vcpu->arch.cpuid_entries, cpuid->nent);
> > >  	if (r) {
> > >  		vcpu->arch.cpuid_nent = 0;
> > >  		goto out;
> > > @@ -940,17 +957,8 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
> > >  struct kvm_cpuid_entry2 *kvm_find_cpuid_entry(struct kvm_vcpu *vcpu,
> > >  					      u32 function, u32 index)
> > >  {
> > > -	struct kvm_cpuid_entry2 *e;
> > > -	int i;
> > > -
> > > -	for (i = 0; i < vcpu->arch.cpuid_nent; ++i) {
> > > -		e = &vcpu->arch.cpuid_entries[i];
> > > -
> > > -		if (e->function == function && (e->index == index ||
> > > -		    !(e->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX)))
> > > -			return e;
> > > -	}
> > > -	return NULL;
> > > +	return cpuid_entry2_find(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent,
> > > +				 function, index);
> > >  }
> > >  EXPORT_SYMBOL_GPL(kvm_find_cpuid_entry);
> > >  
> > 
> > Other than minor note to the commit message, this looks fine, so
> > Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > 
> 
> Thanks!
>