Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive

[Paolo Bonzini <pbonzini@redhat.com>]

On 23/02/2017 11:23, Peter Maydell wrote:
> On 23 February 2017 at 10:10, Paolo Bonzini <pbonzini@redhat.com> wrote:
>> On 23/02/2017 11:02, Peter Maydell wrote:
>>> I'm really not convinced we need DEVICE_HOST_ENDIAN. RAM
>>> areas should be target-endian (you can probably define
>>> "target endianness" as "the endianness that RAM areas have".)
>>
>> This is not RAM.  This is MMIO, backed by a MMIO area in the host.
> 
> Hmm, I see...the naming is a bit unfortunate if it's not RAM.

Yeah, it's called like that because it is backed by a RAMBlock but it
returns false for memory_access_is_direct.

>> The
>> MemoryRegionOps read from the MMIO area (so the data has host
>> endianness) and do not do any further swap:
>>
>>         data = *(uint16_t *)(mr->ram_block->host + addr);
>>
>> Here, the dereference is basically the same as ldl_he_p.
>>
>> If you wanted to make the MemoryRegion use DEVICE_NATIVE_ENDIAN, you'd
>> need to tswap around the access.  Or you can use ldl_le_p and
>> DEVICE_LITTLE_ENDIAN (this is what Yongji's patch open codes), or
>> ldl_be_p and DEVICE_BIG_ENDIAN.  They are all the same in the end.
> 
> Using stl_p &c in a DEVICE_NATIVE_ENDIAN MR would work too, right?
> (This is how all the NATIVE_ENDIAN MRs in exec.c work.)

Yes, it should, as long as the memcpy(...) of {ld,st}*_he_p is compiled
to a single access, which should be the case.

Paolo