Re: [PATCH 04/18] KVM: x86: hyper-v: Introduce VTL awareness to Hyper-V's PV-IPIs

Nicolas Saenz Julienne
2024-09-16T14:52:17Z

On Fri Sep 13, 2024 at 6:02 PM UTC, Sean Christopherson wrote:
> On Sun, Jun 09, 2024, Nicolas Saenz Julienne wrote:
> > HvCallSendSyntheticClusterIpi and HvCallSendSyntheticClusterIpiEx allow
> > sending VTL-aware IPIs. Honour the hcall by exiting to user-space upon
> > receiving a request with a valid VTL target. This behaviour is only
> > available if the VSM CPUID flag is available and exposed to the guest.
> > It doesn't introduce a behaviour change otherwise.
> >
> > User-space is accountable for the correct processing of the PV-IPI
> > before resuming execution.
> >
> > Signed-off-by: Nicolas Saenz Julienne <nsaenz@amazon.com>
> > ---
> > arch/x86/kvm/hyperv.c | 19 ++++++++++++++++++-
> > 1 file changed, 18 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
> > index 42f44546fe79c..d00baf3ffb165 100644
> > --- a/arch/x86/kvm/hyperv.c
> > +++ b/arch/x86/kvm/hyperv.c
> > @@ -2217,16 +2217,20 @@ static void kvm_hv_send_ipi_to_many(struct kvm *kvm, u32 vector,
> >
> > static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
> > {
> > + bool vsm_enabled = kvm_hv_cpuid_vsm_enabled(vcpu);
> > struct kvm_vcpu_hv *hv_vcpu = to_hv_vcpu(vcpu);
> > u64 *sparse_banks = hv_vcpu->sparse_banks;
> > struct kvm *kvm = vcpu->kvm;
> > struct hv_send_ipi_ex send_ipi_ex;
> > struct hv_send_ipi send_ipi;
> > + union hv_input_vtl *in_vtl;
> > u64 valid_bank_mask;
> > + int rsvd_shift;
> > u32 vector;
> > bool all_cpus;
> >
> > if (hc->code == HVCALL_SEND_IPI) {
> > + in_vtl = &send_ipi.in_vtl;
>
> I don't see any value in having a local pointer to a union. Just use send_ipi.in_vtl.

OK, I'll simplify it.

> > if (!hc->fast) {
> > if (unlikely(kvm_read_guest(kvm, hc->ingpa, &send_ipi,
> > sizeof(send_ipi))))
> > @@ -2235,16 +2239,22 @@ static u64 kvm_hv_send_ipi(struct kvm_vcpu *vcpu, struct kvm_hv_hcall *hc)
> > vector = send_ipi.vector;
> > } else {
> > /* 'reserved' part of hv_send_ipi should be 0 */
> > - if (unlikely(hc->ingpa >> 32 != 0))
> > + rsvd_shift = vsm_enabled ? 40 : 32;
> > + if (unlikely(hc->ingpa >> rsvd_shift != 0))
> > return HV_STATUS_INVALID_HYPERCALL_INPUT;
>
> The existing error handling doesn't make any sense to me. Why is this the _only_
> path that enforces reserved bits?

I don't know.

As far as I can tell, the hypercall was introduced in v5 of the TLFS and
already contained the VTL selection bits. Unfortunately the spec doesn't
explicitly state what to do when hv_input_vtl is received from a non-VSM
enabled guest, so I tried to keep the current behaviour for every case
(send_ipi/send_ipi_ex/fast/!fat).

> Regarding the shift, I think it makes more sense to do:
>
> /* Bits 63:40 are always reserved. */
> if (unlikely(hc->ingpa >> 40 != 0))
> return HV_STATUS_INVALID_HYPERCALL_INPUT;
>
> send_ipi.in_vtl.as_uint8 = (u8)(hc->ingpa >> 32);
> if (unlikely(!vsm_enabled && send_ipi.in_vtl.as_uint8))
> return HV_STATUS_INVALID_HYPERCALL_INPUT;
>
> so that it's more obvious exactly what is/isn't reserved when VSM isn't/is enabled.

OK, I agree it's nicer.

> > + in_vtl->as_uint8 = (u8)(hc->ingpa >> 32);
> > sparse_banks[0] = hc->outgpa;
> > vector = (u32)hc->ingpa;
> > }
> > all_cpus = false;
> > valid_bank_mask = BIT_ULL(0);
> >
> > + if (in_vtl->use_target_vtl)
>
> Due to the lack of error checking for the !hc->fast case, this will do the wrong
> thing if vsm_enabled=false.

Yes. I'll fix it.

Thanks,
Nicolas