Re: [PATCH v7 14/14] xen/riscv: Disable SSTC extension and add trap-based CSR probing

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 3/11/26 10:54 AM, Oleksii Kurochko wrote:
> On 3/10/26 10:15 AM, Jan Beulich wrote:
> > On 06.03.2026 17:33, Oleksii Kurochko wrote:
> > > Some RISC-V platforms expose the SSTC extension, but its CSRs are not
> > > properly saved and restored by Xen. Using SSTC in Xen could therefore
> > > lead to unexpected behaviour.
> > And what's wrong with (or what gets in the way of) adding proper
> > saving/restoring? Also, wouldn't a guest use vstimecmp anyway? I.e. what
> > saving/restoring are you talking about here?
> >
> > > To avoid this in QEMU, disable SSTC by passing "sstc=off". On real
> > > hardware, OpenSBI does not provide a mechanism to disable SSTC via the
> > > DTS (riscv,isa or similar property), as it does not rely on that
> > > property to determine extension availability. Instead, it directly
> > > probes the CSR_STIMECMP register.
> > >
> > > Introduce struct trap_info together with the do_expected_trap() handler
> > > to safely probe CSRs. The helper csr_read_allowed() attempts to read a
> > > CSR while catching traps, allowing Xen to detect whether the register
> > > is accessible. This mechanism is used at boot to verify SSTC support 
> > > and
> > > panic if the CSR is not available.
> > >
> > > The trap handling infrastructure may also be reused for other cases
> > > where controlled trap handling is required (e.g. probing instructions
> > > such as HLV*).
> > Hmm, won't you need a more generic way of dealing with traps anyway? See
> > Linux'es _ASM_EXTABLE(). See also comments further down.
> At the moment this approach works for me and I haven't had a need for 
> more
> generic approach. I will look at _ASM_EXTABLE(). I haven't checked yet 
> but
> I assume it will require some extra fixup code in trap handler what looks
> like over complication for the current case, at least.
I checked _ASM_EXTABLE() implementation and so I will need basically provide 
only
for now implementation of EX_TYPE_FIXUP what doesn't look too much and is 
comparable
with the current suggested solution.

~ Oleksii