Re: [RFC PATCH v2 3/5] exec/memattrs: Introduce MemTxAttrs::bus_perm field

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Mon, Aug 23, 2021 at 06:41:55PM +0200, Philippe Mathieu-Daudé wrote:
> Add the 'direct_access' bit to the memory attributes to restrict
> bus master access to ROM/RAM.
> Have read/write accessors return MEMTX_BUS_ERROR if an access is
> restricted and the region is not ROM/RAM ('direct').
> Add corresponding trace events.
> 
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  include/exec/memattrs.h | 21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/include/exec/memattrs.h b/include/exec/memattrs.h
> index 95f2d20d55b..7a94ee75a88 100644
> --- a/include/exec/memattrs.h
> +++ b/include/exec/memattrs.h
> @@ -14,6 +14,13 @@
>  #ifndef MEMATTRS_H
>  #define MEMATTRS_H
>  
> +/* Permission to restrict bus memory accesses. See MemTxAttrs::bus_perm */
> +enum {
> +    MEMTXPERM_UNSPECIFIED   = 0,
> +    MEMTXPERM_UNRESTRICTED  = 1,
> +    MEMTXPERM_RAM_DEVICE    = 2,
> +};
> +
>  /* Every memory transaction has associated with it a set of
>   * attributes. Some of these are generic (such as the ID of
>   * the bus master); some are specific to a particular kind of
> @@ -35,6 +42,19 @@ typedef struct MemTxAttrs {
>      unsigned int secure:1;
>      /* Memory access is usermode (unprivileged) */
>      unsigned int user:1;
> +    /*
> +     * Bus memory access permission.
> +     *
> +     * Some devices (such DMA) might be restricted to only access
> +     * some type of device, such RAM devices. By default memory
> +     * accesses are unspecified (MEMTXPERM_UNSPECIFIED), but could be
> +     * unrestricted (MEMTXPERM_UNRESTRICTED, similar to an allow list)
> +     * or restricted to a type of devices (similar to a deny list).
> +     * Currently only RAM devices can be restricted (MEMTXPERM_RAM_DEVICE).

I don't understand these 3 categories.

MEMTXPERM_UNSPECIFIED means any MemoryRegion can be accessed?

What does MEMTXPERM_UNRESTRICTED mean? How does this differ from
MEMTXPERM_UNSPECIFIED?

What exactly does MEMTXPERM_RAM_DEVICE mean? Maybe that only
MemoryRegions where memory_region_is_ram() is true can be accessed?

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