Re: [PATCH v2 1/3] xen/riscv: fix switch_stack_and_jump() for range beyond 1M

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 6/2/26 1:21 PM, Andrew Cooper wrote:
> On 25/05/2026 2:20 pm, Oleksii Kurochko wrote:
> > The `j` instruction (JAL x0) used in switch_stack_and_jump() is a
> > J-type instruction with only a ±1MB range, and that this can
> > be exceeded in some configurations, causing a linker error:
> >
> >    relocation truncated to fit: R_RISCV_JAL against `<symbol>'
> >
> > Replace `j` with `jr` (JALR x0) via an explicit register, which has
> > unlimited range.
> >
> > Found in a downstream branch when UBSAN instrumentation was enabled.
> >
> > Note that the `tail` instruction looks more natural here, but `jr` is
> > chosen instead to avoid depending on how the assembler expands `tail`
> > and which scratch register it uses (`t1` in GAS), which would need to
> > be listed in the clobber section of `asm volatile`.
> >
> > Fixes: e66003e7be199 ("xen/riscv: introduce setup_initial_pages")
> > Signed-off-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
> > Reviewed-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
> > ---
> > Changes in v2:
> >   - Update the commit message and subject
> > ---
> >   xen/arch/riscv/include/asm/current.h | 10 +++++-----
> >   1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/xen/arch/riscv/include/asm/current.h 
> > b/xen/arch/riscv/include/asm/current.h
> > index 5fbee8182caa..cc004670d18c 100644
> > --- a/xen/arch/riscv/include/asm/current.h
> > +++ b/xen/arch/riscv/include/asm/current.h
> > @@ -51,11 +51,11 @@ DECLARE_PER_CPU(struct vcpu *, curr_vcpu);
> >   #define vcpu_guest_cpu_user_regs(vcpu) \
> >       (&(vcpu)->arch.cpu_info->guest_cpu_user_regs)
> >   
> > -#define switch_stack_and_jump(stack, fn) do {               \
> > -    asm volatile (                                          \
> > -            "mv sp, %0\n"                                   \
> > -            "j " #fn :: "r" (stack), "X" (fn) : "memory" ); \
> > -    unreachable();                                          \
> > +#define switch_stack_and_jump(stack, fn) do {                    \
> > +    asm volatile (                                               \
> > +            "mv sp, %0\n"                                        \
> > +            "jr %1" :: "r" (stack), "r" (fn) : "memory" );       \
> > +    unreachable();                                               \
> >   } while ( false )
> Just as a note, with this simplified again, the \'s can avoid moving, at
> which point the diff becomes a single line.
>
> Can be fixed on commit.
I would be happy with that.

Thanks.

~ Oleksii