On Thu, Mar 04, 2021, Xu, Like wrote:
> Hi Sean,
>
> Thanks for your detailed review on the patch set.
>
> On 2021/3/4 0:58, Sean Christopherson wrote:
> > On Wed, Mar 03, 2021, Like Xu wrote:
> > > @@ -348,10 +352,26 @@ static bool intel_pmu_handle_lbr_msrs_access(struct kvm_vcpu *vcpu,
> > > return true;
> > > }
> > > +/*
> > > + * Check if the requested depth values is supported
> > > + * based on the bits [0:7] of the guest cpuid.1c.eax.
> > > + */
> > > +static bool arch_lbr_depth_is_valid(struct kvm_vcpu *vcpu, u64 depth)
> > > +{
> > > + struct kvm_cpuid_entry2 *best;
> > > +
> > > + best = kvm_find_cpuid_entry(vcpu, 0x1c, 0);
> > > + if (best && depth && !(depth % 8))
> > This is still wrong, it fails to weed out depth > 64.
>
> How come ? The testcases depth = {65, 127, 128} get #GP as expected.
@depth is a u64, throw in a number that is a multiple of 8 and >= 520, and the
"(1ULL << (depth / 8 - 1))" will trigger undefined behavior due to shifting
beyond the capacity of a ULL / u64.
Adding the "< 64" check would also allow dropping the " & 0xff" since the check
would ensure the shift doesn't go beyond bit 7. I'm not sure the cleverness is
worth shaving a cycle, though.
> > Not that this is a hot path, but it's probably worth double checking that the
> > compiler generates simple code for "depth % 8", e.g. it can be "depth & 7)".
>
> Emm, the "%" operation is quite normal over kernel code.
So is "&" :-) I was just pointing out that the compiler should optimize this,
and it did.
> if (best && depth && !(depth % 8))
> 10659: 48 85 c0 test rax,rax
> 1065c: 74 c7 je 10625 <intel_pmu_set_msr+0x65>
> 1065e: 4d 85 e4 test r12,r12
> 10661: 74 c2 je 10625 <intel_pmu_set_msr+0x65>
> 10663: 41 f6 c4 07 test r12b,0x7
> 10667: 75 bc jne 10625 <intel_pmu_set_msr+0x65>
>
> It looks like the compiler does the right thing.
> Do you see the room for optimization ？
>
> >
> > > + return (best->eax & 0xff) & (1ULL << (depth / 8 - 1));
Actually, looking at this again, I would explicitly use BIT() instead of 1ULL
(or BIT_ULL), since the shift must be 7 or less.
> > > +
> > > + return false;
> > > +}
> > > +
>
