Re: [Qemu-devel] [PATCH for-1.4] libi2c-omap: Fix endianness dependency

[Peter Maydell]

On 2 February 2013 18:26, Blue Swirl <address@hidden> wrote:
> On Sat, Feb 2, 2013 at 5:44 PM, Peter Maydell <address@hidden> wrote:
>> On 2 February 2013 17:37, Andreas Färber <address@hidden> wrote:
>>> Am 02.02.2013 17:49, schrieb Peter Maydell:
>>> libqtest.h has no generic endian-aware memread functions unlike Alex,
>>> you or me expected. It reads a sequence of bytes from guest memory and
>>> transmits them one-by-one over the text-based qtest protocol.
>>
>> OK, so this is just busted for accessing devices. The protocol
>> has to have some way of letting you do a 32 bit / 16 bit / 8 bit
>> access (and maybe 64 bit as well while we're here). memread
>> and memwrite are OK for RAM accesses [ie anything you'd be
>> happy to have cached or buffered in a real system] but for
>> memory mapped registers we need to have an equivalent of
>> inb/inw/inl/outb/outw/outl that guarantee to do exactly one
>> access of exactly the required width.
>
> I was also just making a patch (but not so nice as Andreas'). My
> analysis was that qtest.c and libqtest.c just pass the result of
> cpu_physical_memory_rw() as is, with no endian conversion. So the
> result needs to be converted to host CPU order just like Andreas did.
> I think the protocol is OK, my initial reaction was to put byte
> swapping there but that's not right.

No, I think the protocol is broken, even if you ignore the
question of endianness. Consider that a device's MemoryRegion
can have different behaviour depending on whether you access it
as a byte, halfword or word size. cpu_physical_memory_rw() won't
let you make a word size access to an unaligned address.

-- PMM