Re: [PATCH] x86/viridian: use hv_timer_message_payload struct

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 13/11/2025 5:24 pm, Roger Pau Monne wrote:
> diff --git a/xen/arch/x86/hvm/viridian/synic.c 
> b/xen/arch/x86/hvm/viridian/synic.c
> index e6cba7548f1b..6d7b6bd0eda2 100644
> --- a/xen/arch/x86/hvm/viridian/synic.c
> +++ b/xen/arch/x86/hvm/viridian/synic.c
> @@ -327,15 +327,10 @@ bool viridian_synic_deliver_timer_msg(struct vcpu *v, 
> unsigned int sintx,
>      struct viridian_vcpu *vv = v->arch.hvm.viridian;
>      const union hv_synic_sint *vs = &vv->sint[sintx];
>      struct hv_message *msg = vv->simp.ptr;
> -    struct {
> -        uint32_t TimerIndex;
> -        uint32_t Reserved;
> -        uint64_t ExpirationTime;
> -        uint64_t DeliveryTime;
> -    } payload = {
> -        .TimerIndex = index,
> -        .ExpirationTime = expiration,
> -        .DeliveryTime = delivery,
> +    const struct hv_timer_message_payload payload = {
> +        .timer_index = index,
> +        .expiration_time = expiration,
> +        .delivery_time = delivery,

Align these = for readability?

>      };
>  
>      /* Don't assume SIM page to be mapped. */
> @@ -359,8 +354,8 @@ bool viridian_synic_deliver_timer_msg(struct vcpu *v, 
> unsigned int sintx,
>      msg->header.message_flags.msg_pending = 0;
>      msg->header.payload_size = sizeof(payload);
>  
> -    BUILD_BUG_ON(sizeof(payload) > sizeof(msg->u.payload));
> -    memcpy(msg->u.payload, &payload, sizeof(payload));
> +    BUILD_BUG_ON(sizeof(msg->payload.timer) > sizeof(msg->payload.raw));

This BUILD_BUG_ON() was only needed because of the memcpy() between
different types.  With structure assignment, the compiler will tell you
if the type mismatches.

Therefore, it's safe to drop.

Otherwise, Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>