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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).
thanks
-- PMM
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, (continued)
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Paolo Bonzini, 2015/07/02
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/04
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Paolo Bonzini, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Paolo Bonzini, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Peter Maydell, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Peter Maydell, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability,
Peter Maydell <=
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Paolo Bonzini, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Michael S. Tsirkin, 2015/07/06
- Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Paolo Bonzini, 2015/07/06
Re: [Qemu-devel] [PATCH] virtio-pci: implement cfg capability, Gerd Hoffmann, 2015/07/03