Re: [PATCH v1 3/3] KVM: arm64: Add host_hvc hyp event

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Oct 09, 2026 at 01:26:22PM +0100, Fuad Tabba wrote:
> Hi Vincent,
> 
> On Fri, 09 Oct 2026 09:45:29 +0100, Vincent Donnefort
> <vdonnefort@google.com> wrote:
> 
> [...]
> > diff --git a/arch/arm64/include/asm/kvm_hypevents.h b/arch/arm64/include/asm/kvm_hypevents.h
> 
> [...]
> > +HYP_EVENT(host_hvc,
> > +        HE_PROTO(u8 id, u64 ret),
> > +        HE_STRUCT(
> > +                he_field(u8, id)
> > +                he_field(u64, ret)
> > +        ),
> > +        HE_ASSIGN(
> > +                __entry->id = id;
> > +                __entry->ret = ret;
> > +        ),
> > +        HE_PRINTK("id=%u ret=0x%llx", __entry->id, __entry->ret)
> > +);
> 
> id is the index into host_hcall[], so its value depends on that
> table's order. Could this log the SMCCC function ID instead, as
> guest_hvc does? The alignment issue from patch 2 applies here too: the
> format file puts ret at offset 3 instead of 8.

Ack. I thought this might be easier to get a simpler ID. I even contemplated to
stringify the list before convincing myself this wasn't really the hassle.

> 
> [...]
> > diff --git a/arch/arm64/kvm/hyp/nvhe/hyp-main.c b/arch/arm64/kvm/hyp/nvhe/hyp-main.c
> 
> [...]
> > @@ -996,6 +996,8 @@ static void handle_host_hcall(struct kvm_cpu_context *host_ctxt)
> >          cpu_reg(host_ctxt, 0) = SMCCC_RET_SUCCESS;
> >          hfn(host_ctxt);
> >
> > +        trace_host_hvc(id, cpu_reg(host_ctxt, 1));
> > +
> >          return;
> >  inval:
> >          cpu_reg(host_ctxt, 0) = SMCCC_RET_NOT_SUPPORTED;
> 
> Only some of the handlers write a return value to x1. For the others,
> x1 still holds the first argument, e.g. the mmu pointer for
> __kvm_tlb_flush_vmid, so host_hvc's ret field shows that instead.
> Could the field be called x1 rather than ret?

x1 it is. Perhaps I'll rename too for guest_hvc.

> 
> Cheers,
> /fuad

And as always, thanks for reviewing.

-- 
Vincent