Re: [RFC PATCH v6 00/35] KVM: arm64: Add Statistical Profiling Extension (SPE) support

Leo Yan
2025-12-12T11:15:50Z

On Fri, Dec 12, 2025 at 10:18:27AM +0000, Alexandru Elisei wrote:

[...]

> > 3) In the end, the KVM hypervisor pins physical pages on the host
> > stage-1 page table for:
>
> By 'pin' meaning using pin_user_pages(), yes.
>
> >
> > The physical pages are pinned for Guest stage-1 table;
>
> Yes.
>
> > The physical pages are pinned for Guest stage-2 table;
>
> Yes and no. The pages allocated for the stage 2 translation tables are not
> mapped in the host's userspace, they are mapped in the kernel linear address
> space. This means that they are not subject to migration/swap/compaction/etc,
> they will only be reused after KVM frees them.
>
> But that's how KVM manages stage 2 for all VMs, so maybe I misunderstood what
> you were saying.

No, you did not misunderstand. I did not understand stage-2 table
allocation before — it is allocated by KVM, not from user memory via
the VMM.

[...]

> > Due the host might migrate or swap pages, so all the pin operations
> > happen on the host's page table. The pin operations never to be set up
> > in guest's stage-2 table, right?
>
> I'm not sure what you mean.

Never mind. I think you have answered this below (pin user memory via
pin_user_pages() and no matter with stage-2 tables).

> > My understanding is that there are two prominent challenges for SPE
> > virtualization:
> >
> > 1) Allocation: we need to allocate trace buffer with mapping both
> > guest's stage-1 and stage-2 before enabling SPU. (For me, the free
>
> It's the guest responsibility to map the buffer in the guest stage 1 before
> enabling it. When the guest enables the buffer, KVM walks the guest's stage 1
> and if it doesn't find a translation for a buffer guest VA, it will inject a
> profiling buffer management event to the guest, with EC stage 1 data abort.

IIUC, KVM will inject a buffer management interrupt to guest and then
guest driver can detect EC="stage 1 data abort". KVM does not raise a
data abort exception in this case.

> If the buffer was mapped in the guest stage 1 when the guest enabled the buffer,
> but at same point in the future the guest unmaps the buffer from stage 1, the
> statistical profiling unit might encounter a stage 1 data abort when attempting
> to write to memory. If that's the case, the interrupt is taken by the host, and
> KVM will inject the buffer management event back to the guest.

Hmm... just a note, it would be straightforward for guest to directly
respond IRQ for "stage-1 data abort" (TBH, I don't know how to inject
IRQ vs fast-forward IRQ, you could ignore this note until I dig a bit).

Thanks for quick response. The info is quite helpful for me.

Leo