Re: [PATCH v3 07/39] xen/riscv: introduce CPU_NONE

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.09.2026 18:18, Oleksii Kurochko wrote:
> Introduce CPU_NONE, a CPU number which doesn't identify any physical
> CPU, to be used wherever a pCPU has to be recorded but there may be
> none.
> 
> Its first user is vsfile_cpu in struct vimsic_state, which records the
> pCPU whose h/w guest interrupt file backs the vCPU's IMSIC VS-file.
> NR_CPUS has been used there as the value meaning that there is no such
> pCPU, i.e. that the s/w VS-file is in use. NR_CPUS is however an upper
> bound rather than a "no CPU" marker, and it already carries other
> meanings: hartid_to_cpuid() returns it for a hart Xen doesn't know, and
> pcpu_info[] uses it for an entry whose processor_id isn't valid yet.

Yet these could (and imo should, subsequently) entirely plausibly also be
switched to CPU_NONE. IOW I don't think ...

> Use CPU_NONE for vsfile_cpu instead, so that the s/w VS-file case can't
> be mistaken for any of those.

... this is a plausible justification (nor in its re-written form as per
Baptiste's suggestion).

If you want such a distinction, you'll need separate sentinels for every
one such field.

> --- a/xen/arch/riscv/include/asm/config.h
> +++ b/xen/arch/riscv/include/asm/config.h
> @@ -136,6 +136,9 @@
>  #define OPT_CONSOLE_STR "dtuart"
>  #define INVALID_VCPU_ID MAX_VIRT_CPUS
>  
> +/* A CPU number which doesn't identify any physical CPU. */
> +#define CPU_NONE (~0U)
> +
>  /* Linkage for RISCV */
>  #ifdef __ASSEMBLER__
>  #define CODE_FILL /* empty */
> --- a/xen/arch/riscv/include/asm/imsic.h
> +++ b/xen/arch/riscv/include/asm/imsic.h
> @@ -71,7 +71,7 @@ struct vimsic_state {
>       */
>      unsigned int guest_file_id;
>      /*
> -     * s/w IMSIC VS-file -> vsfile_cpu == NR_CPUS
> +     * s/w IMSIC VS-file -> vsfile_cpu == CPU_NONE
>       * h/w IMSIC VS-file -> vsfile_cpu < NR_CPUS
>       */
>      unsigned int vsfile_cpu;

As can be seen from the comment alone, there's now the risk of
CPU_NONE < NR_CPUS, at which point users of the field will be
screwed. I think you want

    BUILD_BUG_ON(CPU_NONE < NR_CPUS);

somewhere.

Jan