Re: [PATCH v2 23/39] xen/riscv: look up the exception table for any trap taken in Xen context

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/18/26 10:44 AM, Baptiste Le Duc wrote:
> > do_trap() consulted the exception table only for CAUSE_ILLEGAL_INSTRUCTION,
> > which covers csr_read_safe() but not the hlv/hlvx sequences reading guest
> > memory: those fault with load/store (guest) page fault causes and would
> > reach do_unexpected_trap() instead of their fixup.
> >
> > Move the lookup ahead of the cause switch, and gate it on the trap having
> > been taken in Xen context and not being an interrupt:
> >
> > - sepc of a trap taken from the guest is a guest VA/PA, which the
> >    guest can point at an address listed in the exception table; Xen would
> >    then act on that entry and, for EX_TYPE_TRAP_INFO, write through a
> >    pointer fully under guest control. Entries are matched by exact address,
> >    so this needs no more than a numerical collision.
> >
> > - an interrupt taken at an address listed in the table would otherwise be
> >    "fixed up" as if the access itself had faulted, silently skipping it and
> >    handing the caller the interrupt's scause as a fault cause.
> >
> > Returning early skips check_for_pcpu_work(), which is correct: that only
> > runs for traps taken from the guest.
> >
> > With that in place a G-stage fault reaching the switch can no longer have
> > been caused by an hlv/hlvx covered by an entry, so anything left must have
> > come from the guest; assert as much.
> What means `assert as much` here? As you referencing any assert in the code,
> because in this patch, I couldn't find any.
It refers to the BUG_ON(!from_guest) in the guest page fault case of 
do_trap(). That BUG_ON() (and the comment above it) ended up in 
"xen/riscv: add guest page fault handling stub" when the series was 
reordered, so this sentence is stale.
I will drop 'assert as much' from commit message.

> > Cache the "trap came from the guest" test in a local, it is now used four
> > times.
> Nit: By reading this sentence, I would expect to have the introduction
> of `from_guest` local here instead of in patch cc2d3b97e8
> xen/riscv: add guest page fault handling stub
`from_guest` is introduced in "xen/riscv: add guest page fault handling 
stub" so it is a stale part of the commit message.
Thanks!

~ Oleksii