Re: [PATCH] KVM: x86: Shove vp_bitmap handling down into sparse_set_to_vcpu_mask()

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 29, 2021, Vitaly Kuznetsov wrote:
> Sean Christopherson <seanjc@google.com> 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.