Re: [PATCH v1 5/6] xen/arm: io: Add a read-const writes-ignored mmio handler

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Tue, 24 Sep 2024, Edgar E. Iglesias wrote:
> From: "Edgar E. Iglesias" <edgar.iglesias@xxxxxxx>
> 
> Add a read-const writes-ignored mmio handler. This is useful
> to for example register background regions that return a fixed
> value instead of raising data aborts.
> 
> Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>

Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
>  xen/arch/arm/include/asm/mmio.h |  2 ++
>  xen/arch/arm/io.c               | 21 +++++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/xen/arch/arm/include/asm/mmio.h b/xen/arch/arm/include/asm/mmio.h
> index 7da542cd79..605620a2f4 100644
> --- a/xen/arch/arm/include/asm/mmio.h
> +++ b/xen/arch/arm/include/asm/mmio.h
> @@ -82,6 +82,8 @@ struct vmmio {
>      struct mmio_handler *handlers;
>  };
>  
> +extern const struct mmio_handler_ops mmio_read_const_writes_ignored;
> +
>  enum io_state try_handle_mmio(struct cpu_user_regs *regs,
>                                mmio_info_t *info);
>  void register_mmio_bg_handler(struct domain *d,
> diff --git a/xen/arch/arm/io.c b/xen/arch/arm/io.c
> index 934a2ad2b9..8ab0435afc 100644
> --- a/xen/arch/arm/io.c
> +++ b/xen/arch/arm/io.c
> @@ -20,6 +20,27 @@
>  
>  #include "decode.h"
>  
> +/*
> + * Reusable mmio handler useful as background while waiting for IOREQ.
> + * Register with priv as default read value. Writes ignored.
> + */
> +static int bg_read(struct vcpu *v, mmio_info_t *info, register_t *r, void 
> *priv)
> +{
> +    *r = (uintptr_t) priv;
> +    return 1;
> +}
> +
> +static int bg_write(struct vcpu *v, mmio_info_t *info, register_t r, void 
> *priv)
> +{
> +    return 1;
> +}
> +
> +/* Read const value (from priv), writes ignored.  */
> +const struct mmio_handler_ops mmio_read_const_writes_ignored = {
> +    .read = bg_read,
> +    .write = bg_write,
> +};
> +
>  static enum io_state handle_read(const struct mmio_handler *handler,
>                                   struct vcpu *v,
>                                   mmio_info_t *info)
> -- 
> 2.43.0
>