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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

~Andrew