Re: [PATCH v9 29/30] KVM: arm64: Add selftest event support to nVHE/pKVM hyp

[Vincent Donnefort <vdonnefort@google.com>]

On Fri, Jan 23, 2026 at 12:14:34PM +0000, Vincent Donnefort wrote:
> On Wed, Jan 07, 2026 at 03:40:22PM +0000, Marc Zyngier wrote:
> > On Tue, 02 Dec 2025 09:36:22 +0000,
> > Vincent Donnefort <vdonnefort@google.com> wrote:
> > > 
> > > Add a selftest event that can be triggered from a `write_event` tracefs
> > > file. This intends to be used by trace remote selftests.
> > > 
> > > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > > 
> > > diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
> > > index 37c015b54ff6..e2de7ad64938 100644
> > > --- a/arch/arm64/include/asm/kvm_asm.h
> > > +++ b/arch/arm64/include/asm/kvm_asm.h
> > > @@ -96,6 +96,7 @@ enum __kvm_host_smccc_func {
> > >  	__KVM_HOST_SMCCC_FUNC___tracing_update_clock,
> > >  	__KVM_HOST_SMCCC_FUNC___tracing_reset,
> > >  	__KVM_HOST_SMCCC_FUNC___tracing_enable_event,
> > > +	__KVM_HOST_SMCCC_FUNC___tracing_write_event,
> > >  };
> > >  
> > >  #define DECLARE_KVM_VHE_SYM(sym)	extern char sym[]
> > > diff --git a/arch/arm64/include/asm/kvm_hypevents.h b/arch/arm64/include/asm/kvm_hypevents.h
> > > index 268b3cd7a1b2..c276fd28e0bf 100644
> > > --- a/arch/arm64/include/asm/kvm_hypevents.h
> > > +++ b/arch/arm64/include/asm/kvm_hypevents.h
> > > @@ -42,4 +42,15 @@ HYP_EVENT(hyp_exit,
> > >  	),
> > >  	HE_PRINTK("reason=%s", __hyp_enter_exit_reason_str(__entry->reason))
> > >  );
> > > +
> > > +HYP_EVENT(selftest,
> > > +	HE_PROTO(u64 id),
> > > +	HE_STRUCT(
> > > +		he_field(u64, id)
> > > +	),
> > > +	HE_ASSIGN(
> > > +		__entry->id = id;
> > > +	),
> > > +	RE_PRINTK("id=%llu", __entry->id)
> > 
> > Not strictly related to this patch, but I find that the trace itself
> > lacks context. For example:
> > 
> > [001]	  323.847422: hyp_enter reason=hvc
> > [001]	  323.847423: hyp_exit reason=eret_host
> > [001]	  323.847688: hyp_enter reason=hvc
> > [001]	  323.847688: hyp_exit reason=eret_host
> > [001]	  323.847706: hyp_enter reason=hvc
> > [001]	  323.847707: hyp_exit reason=eret_host
> > [001]	  323.847722: hyp_enter reason=hvc
> > [001]	  323.847723: hyp_exit reason=eret_host
> > 
> > That's all fine as long as I'm dealing with a single guest, or even
> > with a single vcpu. Trying to trace multiple guests, or even multiple
> > vcpus makes the whole thing completely unusable, because I have no
> > idea what I'm looking at.
> > 
> > To make this useful, having some context provided by the host really
> > is required.
> 
> I could add to the event header the VM pid related to the currently loaded vCPU
> (if any). I can access it easily with host_kvm->userspace_pid. WDYS?

Or actually directly kvm_vcpu->pid?

> 
> > 
> > Thanks,
> > 
> > 	M.
> > 
> > -- 
> > Without deviation from the norm, progress is not possible.