Re: [Qemu-devel] [PATCH 1/4] ahci: Do not ignore memory access read size

[Eric Blake <eblake@redhat.com>]

[-- Attachment #1: Type: text/plain, Size: 1147 bytes --]

On 06/15/2015 05:44 PM, John Snow wrote:

>> Note that while this returns the desired 16 or 32 bits in the low
>> order side of the result, it does not guarantee that the remaining
>> upper bytes are all 0.  I don't know if it matters to callers, or
>> even what real hardware does, but you may want to mask things at
>> both return statements, to guarantee a stable result limited to
>> size bytes of information rather than leaking nearby bytes from the
>> rest of the registers being read.
> 
> 
> I believe the masking is handled by the memory system in general, see
> memory_region_read_accessor, which masks the returned uint64_t with an
> appropriate value set earlier by access_with_adjusted_size:
> 
> access_mask = -1ULL >> (64 - access_size * 8);

Good, the caller takes care of it.

> 
> So we're probably OK here, but I can leave a comment if you think it's
> too hand-wavey.

Yeah, always nicer to state our assumption that we rely on the caller to
truncate the result to the desired access size.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 604 bytes --]