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

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive
Date: Thu, 23 Feb 2017 11:34:55 +0000

On 23 February 2017 at 10:33, Paolo Bonzini <address@hidden> wrote:
> On 23/02/2017 11:23, Peter Maydell wrote:
>> On 23 February 2017 at 10:10, Paolo Bonzini <address@hidden> 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.

We should probably update the doc comment to note that the
pointer is to host-endianness memory (and that this is not
like normal RAM which is target-endian)...

>>> 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.

...and whichever of these approaches we take, we should have
a comment which notes that we are converting from the host
endianness memory to the endianness specified by the MemoryRegion
endianness attribute.

-- PMM

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