Re: [PATCH 1/4] x86/guest: move allocation of Xen upcall vector to init code

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 19/11/2025 10:50 am, Jan Beulich wrote:
> There's no need to do this every time init_evtchn() is called. Just do it
> once when setting up CPU0. Drop the assertion as well, as
> alloc_hipriority_vector() (called by alloc_direct_apic_vector()) uses more
> restrictive BUG_ON() anyway. Then evtchn_upcall_vector can also validly
> become ro-after-init, just that it needs to move out of init_evtchn().
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/arch/x86/guest/xen/xen.c
> +++ b/xen/arch/x86/guest/xen/xen.c
> @@ -233,16 +233,12 @@ static void cf_check xen_evtchn_upcall(v
>      ack_APIC_irq();
>  }
>  
> +static uint8_t __ro_after_init evtchn_upcall_vector;
> +
>  static int init_evtchn(void)
>  {
> -    static uint8_t evtchn_upcall_vector;
>      int rc;
>  
> -    if ( !evtchn_upcall_vector )
> -        alloc_direct_apic_vector(&evtchn_upcall_vector, xen_evtchn_upcall);
> -
> -    ASSERT(evtchn_upcall_vector);
> -
>      rc = xen_hypercall_set_evtchn_upcall_vector(this_cpu(vcpu_id),
>                                                  evtchn_upcall_vector);
>      if ( rc )
> @@ -293,6 +289,8 @@ static void __init cf_check setup(void)
>                 XEN_LEGACY_MAX_VCPUS);
>      }
>  
> +    alloc_direct_apic_vector(&evtchn_upcall_vector, xen_evtchn_upcall);
> +
>      BUG_ON(init_evtchn());
>  }
>  
>

This patch is fine, but it would be nicer to split init_evtchn() into
bsp_init_evtchn() and percpu_init_evtchn().

Just out of context in init_evtchn(), there's a check for CPU0 that also
ought to move into bsp_init_evtchn() (and therefore into __init), at
which point the percpu simplifies to a single hypercall, and we keep
subsystem specifics out of setup().

~Andrew