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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 12:43:37 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0 |
On 23/02/2017 12:34, Peter Maydell wrote:
> 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)...
I wouldn't call it host-endianness memory, and I disagree that normal
RAM is target-endian---in both cases it's just a bunch of bytes.
However, the access done by the MemoryRegionOps callbacks needs to match
the endianness declared by the MemoryRegionOps themselves.
Paolo
>>>> 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.
>
> thanks
> -- PMM
>
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, (continued)
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Alexey Kardashevskiy, 2017/02/22
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Peter Maydell, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Paolo Bonzini, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, Yongji Xie, 2017/02/23
- Re: [Qemu-devel] [PATCH] memory: make ram device read/write endian sensitive, David Gibson, 2017/02/23