Re: [RFC PATCH v3 3/3] softmmu/physmem: Introduce MemTxAttrs::memory field and MEMTX_BUS_ERROR

[Peter Maydell]

On Wed, 15 Dec 2021 at 18:24, Philippe Mathieu-Daudé <philmd@redhat.com> wrote:
>
> Add the 'memory' bit to the memory attributes to restrict bus
> controller accesses to memories.
>
> Introduce flatview_access_allowed() to check bus permission
> before running any bus transaction.
>
> Have read/write accessors return MEMTX_BUS_ERROR if an access is
> restricted.
>
> There is no change for the default case where 'memory' is not set.
>
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  include/exec/memattrs.h |  9 +++++++++
>  softmmu/physmem.c       | 43 +++++++++++++++++++++++++++++++++++++++--
>  2 files changed, 50 insertions(+), 2 deletions(-)
>
> diff --git a/include/exec/memattrs.h b/include/exec/memattrs.h
> index 95f2d20d55b..f0063583ee2 100644
> --- a/include/exec/memattrs.h
> +++ b/include/exec/memattrs.h
> @@ -35,6 +35,14 @@ typedef struct MemTxAttrs {
>      unsigned int secure:1;
>      /* Memory access is usermode (unprivileged) */
>      unsigned int user:1;
> +    /*
> +     * Bus interconnect and peripherals can access anything (memories,
> +     * devices) by default. By setting the 'memory' bit, bus transaction
> +     * are restricted to "normal" memories (per the AMBA documentation)
> +     * versus devices. Access to devices will be logged and rejected
> +     * (see MEMTX_BUS_ERROR).
> +     */
> +    unsigned int memory:1;
>      /* Requester ID (for MSI for example) */
>      unsigned int requester_id:16;
>      /* Invert endianness for this page */
> @@ -66,6 +74,7 @@ typedef struct MemTxAttrs {
>  #define MEMTX_OK 0
>  #define MEMTX_ERROR             (1U << 0) /* device returned an error */
>  #define MEMTX_DECODE_ERROR      (1U << 1) /* nothing at that address */
> +#define MEMTX_BUS_ERROR         (1U << 2) /* bus returned an error */

This is kind of odd naming, because MEMTX_DECODE_ERROR already means
"bus/interconnect returned an error" and it generally translates
into what at the OS level gets called a "bus error"...

-- PMM