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

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 12/17/21 23:34, Peter Maydell wrote:
> 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"...

MEMTX_DECODE_ERROR is "nothing at that address". We want a name
for "there is something, but you don't have access to it".
Maybe MEMTX_ILLEGAL_ERROR?