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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive
Date: Thu, 23 Feb 2017 11:33:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

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.

>> 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.


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