Re: [PATCH 1/4] KVM: MMU: fix permission_fault()

[Xiao Guangrong]

On 03/25/2016 09:35 PM, Paolo Bonzini wrote:
> On 25/03/2016 14:19, Xiao Guangrong wrote:
> > kvm-unit-tests complained about the PFEC is not set properly, e.g,:
> > test pte.rw pte.d pte.nx pde.p pde.rw pde.pse user fetch: FAIL: error code 15
> > expected 5
> > Dump mapping: address: 0x123400000000
> > ------L4: 3e95007
> > ------L3: 3e96007
> > ------L2: 2000083
>
> What's the command line for the reproducer?

QEMU=/home/eric/qemu/x86_64-softmmu/qemu-system-x86_64 ./x86-run x86/access.flat

> > diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> > index b70df72..81bffd1 100644
> > --- a/arch/x86/kvm/mmu.h
> > +++ b/arch/x86/kvm/mmu.h
> > @@ -154,7 +154,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> >   				  unsigned pfec)
> >   {
> >   	int cpl = kvm_x86_ops->get_cpl(vcpu);
> > -	unsigned long rflags = kvm_x86_ops->get_rflags(vcpu);
> > +	unsigned long errcode, rflags = kvm_x86_ops->get_rflags(vcpu);
> >
> >   	/*
> >   	 * If CPL < 3, SMAP prevention are disabled if EFLAGS.AC = 1.
> > @@ -175,7 +175,7 @@ static inline u8 permission_fault(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu,
> >   	bool fault = (mmu->permissions[index] >> pte_access) & 1;
> >
> >   	WARN_ON(pfec & (PFERR_PK_MASK | PFERR_RSVD_MASK));
> > -	pfec |= PFERR_PRESENT_MASK;
> > +	errcode = PFERR_PRESENT_MASK;
>
> So is this patch doing the same as "KVM: MMU: precompute page fault
> error code"?  It was necessary after all. :)

Sorry for my mistake... I missed the logic you changed :(

I still prefer to calculating the error code on the fault path which is rare, or
think a way to encapsulate it to permission_fault()...