Re: [PATCH v2 05/11] x86/vioapic: switch to use the EOI callback mechanism

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.09.2020 12:41, Roger Pau Monne wrote:
> --- a/xen/arch/x86/hvm/vlapic.c
> +++ b/xen/arch/x86/hvm/vlapic.c
> @@ -189,7 +189,7 @@ void vlapic_set_irq_callback(struct vlapic *vlapic, 
> uint8_t vec, uint8_t trig,
>  
>      if ( hvm_funcs.update_eoi_exit_bitmap )
>          alternative_vcall(hvm_funcs.update_eoi_exit_bitmap, target, vec,
> -                          trig || callback);
> +                          callback);

There's a shortcoming in the alternative call framework which I
see no way to eliminate but which makes it necessary to use
!!callback here. Otherwise, if the callback happens to sit on a
256-byte boundary (low address byte zero), you'll pass false
when you mean true. (The original use, i.e. prior to patch 3,
of just "trig" was sufficiently okay, because the parameter
- despite being u8 - is effectively used as a boolean by the
callers iirc.)

Or perhaps the best thing is to require wrappers for all hooks
taking bool parameters, because then the necessary conversion
will be done when calling the wrapper.

Jan