Re: [PATCH v2 09/39] xen/riscv: implement virtual APLIC MMIO emulation

[SeungJu Cheon <suunj1331@xxxxxxxxx>]

On Thu, Sep 10, 2026 at 04:24:06PM +0200, Oleksii Kurochko wrote:
[...]
> In v3 I'll (a) stop using ->processor and take the (guest_file_id,
> vsfile_cpu) pair, which imsic_update_state() updates atomically under
> vsfile_lock, and (b) do the snapshot plus the h/w TARGET write under
> aplic.lock, which aplic_reconfigure_target() also holds. As
> imsic_update_state() completes before aplic_reconfigure_target() (also that
> could be checked in this patch series and is introduced a little bit later.
> Probably I have to re-order some patches again) takes the lock, the emulated
> write either happens before the scan (and gets fixed up, or skipped as
> already correct) or after it (and sees the new location).
> 
> Any better option I have now?

Unless I am missing something, the snapshot also needs to handle the
case where the target vCPU has not been attached yet:
vcpu_guest_file_id() returns zero until the vCPU has gone through
imsic_vsfile_attach(), i.e. until it is scheduled for the first time,
and vsfile_cpu is NR_CPUS until then.

With the current code, a write targeting such a vCPU makes
aplic_msi_target_gen() program Guest Index 0 into the physical APLIC
target register. According to AIA section 4.5.16, Guest Index 0 selects
the hart's supervisor-level interrupt file rather than a VS-level guest
interrupt file. Could this cause the MSI to be delivered to Xen's own
interrupt file with the EIID supplied by the guest?

I also could not find where such a target would be updated once the
VS-file is attached. imsic_migrate_vcpu() reprograms the relevant
targets during migration, but the initial imsic_vsfile_attach() path
does not appear to replay targets which were written before the
attachment.

Whether a write targeting an unattached vCPU should be supported seems
like a separate question. Independently of that choice, would it make
sense to avoid programming the physical TARGET register while
guest_file_id is zero? The virtual target could either be rejected, or
retained in the shadow target[] and programmed once the VS-file is
attached.

Thanks,
SeungJu