On 02/12/2012 05:06 PM, Michael S. Tsirkin wrote: > > It's really really simple: > > > > If the device spec says "big endian, specify DEVICE_BIG_ENDIAN, and > > treat the data naturally in the callback. > > If the device spec says "little endian, specify DEVICE_LITTLE_ENDIAN, > > and treat the data naturally in the callback. > > > > That's it. > > OKay, but I'm sure your API does not go read the spec, so > we should not base the description on that :) > Right? > > So I think the following is right? > > > commit 02aa79aac9bec1c8c17d1b7b5405b59b649dfdb9 > Author: Michael S. Tsirkin <m...@redhat.com> > Date: Wed Feb 8 17:16:35 2012 +0200 > > docs: memory.txt document the endian field > > This is an attempt to document the endian > field in memory API. As this is a confusing topic, > add some examples. > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com> > > diff --git a/docs/memory.txt b/docs/memory.txt > index 5bbee8e..9132c86 100644 > --- a/docs/memory.txt > +++ b/docs/memory.txt > @@ -170,3 +170,48 @@ various constraints can be supplied to control how these > callbacks are called: > - .old_portio and .old_mmio can be used to ease porting from code using > cpu_register_io_memory() and register_ioport(). They should not be used > in new code. > +- .endianness; specifies the device endian-ness, which affects > + the handling of the value parameter passed from guest to write > + and returned to guest from read callbacks, as follows: > + void write(void *opaque, target_phys_addr_t addr, > + uint64_t value, unsigned size) > + uint64_t read(void *opaque, target_phys_addr_t addr, > + unsigned size) > + value is always passed in the natural host format, > + low size bytes in value are set, the rest are zero padded > + on input and ignored on output. > + Legal values for endian-ness are: > + DEVICE_NATIVE_ENDIAN - The value is left in the format used by guest. > + Note that although this is typically a fixed format as > + guest drivers take care of endian conversions, > + if host endian-ness does not match the device this will > + result in "mixed endian" since the data is always > + stored in low bits of value. > + > + To handle this data, on write, you typically need to first > + convert to the appropriate type, removing the > + padding. On read, handle the data in the appropriate > + type and then convert to uint64_t, padding with leading zeroes.
No. Data is converted from guest endian to host endian on write (vice versa on read). This works if the device endianness matches the guest endianness. > + > + DEVICE_LITTLE_ENDIAN - The value is assumed to be > + endian, and is converted to host endian. > + DEVICE_BIG_ENDIAN - The value is assumed to be > + big endian, and is converted to host endian. Yes. > + > + As an example, consider a little endian guest writing a 32 bit > + value 0x12345678 into an MMIO register, on a big endian host. > + The value passed to the write callback is documented below: > + > + DEVICE_NATIVE_ENDIAN - value = 0x0000000087654321 > + Explanation: write callback will get the high bits > + in value set to 0, and low bits set to data left > + as is, that is in little endian format. No, you'll see 0x12345678, same as DEVICE_LITTLE_ENDIAN. > + DEVICE_LITTLE_ENDIAN - value = 0x0000000012345678 > + Explanation: the write callback will get the high bits > + in value set to 0, and low bits set to data in big endian > + format. > + DEVICE_BIG_ENDIAN - value = 0x0000000087654321 > + Explanation: the write callback will get the high bits > + in value set to 0, and low bits set to data in little endian > + format. > + Right value, wrong explanation. The value is still in big endian format. -- error compiling committee.c: too many arguments to function
