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Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability

From: Peter Maydell
Subject: Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability
Date: Mon, 6 Jul 2015 12:50:43 +0100

On 6 July 2015 at 11:31, Michael S. Tsirkin <address@hidden> wrote:
> On Mon, Jul 06, 2015 at 11:04:24AM +0100, Peter Maydell wrote:
>> On 6 July 2015 at 11:03, Michael S. Tsirkin <address@hidden> wrote:
>> > On Mon, Jul 06, 2015 at 10:11:18AM +0100, Peter Maydell wrote:
>> >> But address_space_rw() is just the "memcpy bytes to the
>> >> target's memory" operation -- if you have a pile of bytes
>> >> then there are no endianness concerns. If you don't have
>> >> a pile of bytes then you need to know the structure of
>> >> the data you're DMAing around, and you should probably
>> >> have a loop doing things with the specify-the-width functions.
>> > Absolutely. But what if DMA happens to target another device
>> > and not memory? Device needs some endian-ness so it needs
>> > to be converted to that.
>> Yes, and address_space_rw() already deals with conversion to
>> that device's specified endianness.

> Yes, but incorrectly if target endian != host endian.
> For example, LE target and LE device on BE host.

Having walked through the code, got confused, talked to
bonzini on IRC about it and got unconfused again, I believe
we do get this correct.

 * address_space_rw() takes a pointer to a pile of bytes
 * if the destination is RAM, we just memcpy them (because
   guest RAM is also a pile of bytes)
 * if the destination is a device, then we read a value
   out of the pile of bytes at whatever width the target
   device can handle. The functions we use for this are
   ldl_q/ldl_p/etc, which do "load target endianness"
   (ie "interpret this set of 4 bytes as if it were an
   integer in the target-endianness") because the API of
   memory_region_dispatch_write() is that it takes a uint64_t
   data whose contents are the value to write in target
   endianness order. (This is regrettably undocumented.)
 * memory_region_dispatch_write() then calls adjust_endianness(),
   converting a target-endian value to the endianness the
   device says it requires
 * we then call the device's read/write functions, whose API
   is that they get a value in the endianness they asked for.

> IO callbacks always get a native endian format so they expect to get
> byte 0 of the buffer in MSB on this host.

IO callbacks get the format they asked for (which might
be BE, LE or target endianness). They will get byte 0 of
the buffer in the MSB if they said they were BE devices
(or if they said they were target-endian on a BE target).

-- PMM

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