Re: [PATCH v8 22/28] KVM: arm64: Add trace remote for the pKVM hyp

[Vincent Donnefort <vdonnefort@google.com>]

On Wed, Nov 19, 2025 at 05:31:30PM +0000, Marc Zyngier wrote:
> On Fri, 07 Nov 2025 09:38:34 +0000,
> Vincent Donnefort <vdonnefort@google.com> wrote:
> > 
> > When running with KVM protected mode, the hypervisor is able to generate
> > events into tracefs compatible ring-buffers. Create a trace remote so
> > the kernel can read those buffers.
> > 
> > This currently doesn't provide any event support which will come later.
> > 
> > Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
> > 
> > diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
> > index 580426cdbe77..64db254f0448 100644
> > --- a/arch/arm64/kvm/Kconfig
> > +++ b/arch/arm64/kvm/Kconfig
> > @@ -87,6 +87,7 @@ config PKVM_TRACING
> >  	bool
> >  	depends on KVM
> >  	depends on TRACING
> > +	select TRACE_REMOTE
> >  	select SIMPLE_RING_BUFFER
> >  	default y
> >

[...]

> > +static void hyp_trace_buffer_free_bpages_backing(struct hyp_trace_buffer *trace_buffer)
> > +{
> > +	free_pages_exact((void *)trace_buffer->desc->bpages_backing_start,
> > +			 trace_buffer->desc->bpages_backing_size);
> > +}
> > +
> > +static int __load_page(unsigned long va)
> > +{
> > +	return kvm_call_hyp_nvhe(__pkvm_host_share_hyp, virt_to_pfn((void *)va), 1);
> > +}
> 
> I struggle a bit with the nomenclature here. Why is that called
> "load"? Surely this is a "map" operation, right? Is that because this
> is called at "vcpu load" time? Something else?

I called "load" the operation of getting the tracing buffer ready. But for this
implementation specific part, I can use map/unmap here. (same for
hyp_trace_buffer_map_pages/hyp_trace_buffer_map_pages)

> 
> Also, how is this working without pKVM, in a normal nVHE environment?
> Being able to trace in nVHE is a basic requirement, and I don't see
> how this works here.

I can probably make it work with nVHE as well.

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