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Re: [Qemu-devel] [RFC 1/5] memory: Define API for MemoryRegionOps to tak

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [RFC 1/5] memory: Define API for MemoryRegionOps to take attrs and return status
Date: Fri, 27 Mar 2015 14:16:53 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.5.0

On 27/03/2015 13:32, Edgar E. Iglesias wrote:
>>> Is this related to masters relying on the memory frameworks magic
>>> handling of unaliged accesses?
>> Not necessarily, you can get the same just by doing a large write that
>> spans multiple MemoryRegions.  See the loop in address_space_rw.
> Right, this is another case of "magic" memory handling that allows masters
> to issue unnatural transactions and rely on the memory framework to
> split things up.
> In these cases aren't the masters trading accuracy (including error
> handling accuracy) for performance or model simplicity?

Yes.  There are no "natural" transactions beyond 32 or 64-bit accesses.

> It could maybe be useful to have a flag so masters can say one of the
> following (could be encoded in the memattrs):
> 1. Stop at first error and return.
> 2. Keep going after errors and give me the OR result of all errors.

It could just be a length pointer in the same vein as
address_space_map's.  If NULL, keep going.  If not NULL, stop and return.


> For 1 to be useful, I think it would have to be combined with some
> kind of return info that can point out where in the magic splitting
> of large or unaligned transactions that things went wrong.
> Probably overkill at the moment...
> Cheers,
> Edgar

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