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

[Jan Beulich <jbeulich@suse.com>]

On 27.08.2026 17:21, Oleksii Kurochko 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.
> 
> Cache the "trap came from the guest" test in a local, it is now used four
> times.
> 
> Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

Acked-by: Jan Beulich <jbeulich@suse.com>