Re: [PATCH v6 1/3] memory: drop guest writes to read-only ram device regions

[Paolo Bonzini]

On 25/05/20 03:18, Yan Zhao wrote:
> On Thu, May 21, 2020 at 04:38:47PM +0200, Paolo Bonzini wrote:
>> On 30/04/20 11:40, Peter Maydell wrote:
>>>> This does not "drop" a write to a r/o region -- it causes it to generate
>>>> whatever the guest architecture's equivalent of a bus error is (eg data
>>>> abort on Arm).
>>
>>
>>> More generally, this change seems a bit odd: currently we do not
>>> check the mr->readonly flag here, but in general guests don't get
>>> to write to ROM areas. Where is that check currently done
>>
>> Writes to ROM are directed to mr->ops unassigned_mem_ops.  Because _all_
>> ram-device reads and writes go through the ops, for ram-device we have
>> to stick the check for mr->readonly in the ops.
>>
>> On one hand, I was quite surprised to see that unassigned_mem_write does
>> not return MEMTX_ERROR now that I looked at it.
>>
>> On the other hand, we should use MEMTX_ERROR in patch 2 as well, if we
>> decide it's the way to go.
>>
>> (Sorry Yan for the late response).
>>
> hi Paolo,
> thanks for your reply and never mind :)
> 
> But there's one thing I just can't figure out the reason and eagerly need
> your guide.
> 
> why do we have to convert all .write operations to .write_with_attrs and
> return MEMTX_ERROR? because of the handling of writes to read-only region?

Not all of them, only those that need to return MEMTX_ERROR.  I would
like some guidance from Peter as to whether (or when) reads from ROMs
should return MEMTX_ERROR.  This way, we can use that information to
device  what the read-only ram-device regions should do.

Paolo