On Fri, Oct 29, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@xxxxxxxxxx> writes:
> > There's a smoke test for this in selftests, but it's not really all that
> > interesting. It took me over an hour and a half just to get a Linux guest
> > to hit the relevant flows. Most of that was due to QEMU 5.1 bugs (doesn't
> > advertise HYPERCALL MSR by default)
>
> This should be fixed already, right?
Yeah, it's fixed in more recent versions. That added to the confusion; the local
copy of QEMU source I was reading didn't match the binary I was using. Doh.
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 4f15c0165c05..80018cfab5c7 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -1710,31 +1710,36 @@ int kvm_hv_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata, bool host)
> > return kvm_hv_get_msr(vcpu, msr, pdata, host);
> > }
> >
> > -static __always_inline unsigned long *sparse_set_to_vcpu_mask(
> > - struct kvm *kvm, u64 *sparse_banks, u64 valid_bank_mask,
> > - u64 *vp_bitmap, unsigned long *vcpu_bitmap)
> > +static void sparse_set_to_vcpu_mask(struct kvm *kvm, u64 *sparse_banks,
> > + u64 valid_bank_mask, unsigned long *vcpu_mask)
> > {
> > struct kvm_hv *hv = to_kvm_hv(kvm);
> > + bool has_mismatch = atomic_read(&hv->num_mismatched_vp_indexes);
> > + u64 vp_bitmap[KVM_HV_MAX_SPARSE_VCPU_SET_BITS];
> > struct kvm_vcpu *vcpu;
> > int i, bank, sbank = 0;
> > + u64 *bitmap;
> >
> > - memset(vp_bitmap, 0,
> > - KVM_HV_MAX_SPARSE_VCPU_SET_BITS * sizeof(*vp_bitmap));
> > + BUILD_BUG_ON(sizeof(vp_bitmap) >
> > + sizeof(*vcpu_mask) * BITS_TO_LONGS(KVM_MAX_VCPUS));
> > +
> > + /* If vp_index == vcpu_idx for all vCPUs, fill vcpu_mask directly. */
> > + if (likely(!has_mismatch))
> > + bitmap = (u64 *)vcpu_mask;
> > +
> > + memset(bitmap, 0, sizeof(vp_bitmap));
>
> ... but in the unlikely case has_mismatch == true 'bitmap' is still
> uninitialized here, right? How doesn't it crash?
I'm sure it does crash. I'll hack the guest to actually test this. More below.
> > for_each_set_bit(bank, (unsigned long *)&valid_bank_mask,
> > KVM_HV_MAX_SPARSE_VCPU_SET_BITS)
> > - vp_bitmap[bank] = sparse_banks[sbank++];
> > + bitmap[bank] = sparse_banks[sbank++];
> >
> > - if (likely(!atomic_read(&hv->num_mismatched_vp_indexes))) {
> > - /* for all vcpus vp_index == vcpu_idx */
> > - return (unsigned long *)vp_bitmap;
> > - }
> > + if (likely(!has_mismatch))
> > + return;
> >
> > - bitmap_zero(vcpu_bitmap, KVM_MAX_VCPUS);
> > + bitmap_zero(vcpu_mask, KVM_MAX_VCPUS);
> > kvm_for_each_vcpu(i, vcpu, kvm) {
> > if (test_bit(kvm_hv_get_vpindex(vcpu), (unsigned long *)vp_bitmap))
>
> 'vp_bitmap' also doesn't seem to be assigned to anything, I'm really
> confused :-(
>
> Didn't you accidentally mix up 'vp_bitmap' and 'bitmap'?
No, bitmap was supposed to be initialized as:
if (likely(!has_mismatch))
bitmap = (u64 *)vcpu_mask;
else
bitmap = vp_bitmap;
The idea being that the !mismatch case sets vcpu_mask directly, and the mismatch
case sets vp_bitmap and then uses that to fill vcpu_mask.
