On Sun, Feb 12, 2012 at 03:55:20PM +0200, Avi Kivity wrote:
> On 02/12/2012 03:47 PM, Michael S. Tsirkin wrote:
> > On Sun, Feb 12, 2012 at 03:02:11PM +0200, Avi Kivity wrote:
> > > On 02/12/2012 02:52 PM, Michael S. Tsirkin wrote:
> > > > This is an attempt to document the endian
> > > > field in memory API. As this is a confusing topic,
> > > > it's best to make the text as explicit as possible.
> > > >
> > > > Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
> > > > ---
> > > > docs/memory.txt | 28 ++++++++++++++++++++++++++++
> > > > 1 files changed, 28 insertions(+), 0 deletions(-)
> > > >
> > > > diff --git a/docs/memory.txt b/docs/memory.txt
> > > > index 5bbee8e..ff92b52 100644
> > > > --- a/docs/memory.txt
> > > > +++ b/docs/memory.txt
> > > > @@ -170,3 +170,31 @@ 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 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)
> > > > + Legal values are:
> > > > + DEVICE_NATIVE_ENDIAN - Callbacks accept and return value in
> > > > + host endian format. This makes it possible to do
> > > > + math on values without type conversions.
> > > > + Low size bytes in value are set, the rest are zero padded
> > > > + on input and ignored on output.
> > > > + DEVICE_LITTLE_ENDIAN - Callbacks accept and return value
> > > > + in little endian format. This is appropriate
> > > > + if you need to directly copy the data into device memory,
> > > > + and the device programming interface is little endian
> > > > + (true for most pci devices).
> > > > + First size bytes in value are set, the rest are zero padded
> > > > + on input and ignored on output.
> > > > + DEVICE_BIG_ENDIAN - Callbacks accept and return value
> > > > + in big endian format.
> > > > + in little endian format. This is appropriate
> > > > + if you need to directly copy the data into device memory,
> > > > + and the device programming interface is big endian
> > > > + (true e.g. for some system devices on big endian
> > > > architectures).
> > > > + Last size bytes in value are set, the rest are zero padded
> > > > + on input and ignored on output.
> > >
> > > This is wrong. Callback data is always in host endianness. Device
> > > endianness is about the device.
> > >
> > > For example, DEVICE_BIG_ENDIAN means that the device expects data in big
> > > endian format. Qemu assumes the guest OS writes big endian data to the
> > > device, so it swaps from big endian to host endian before calling the
> > > callback. Similarly it will swap from host endian to big endian on read.
> > >
> > > DEVICE_NATIVE_ENDIAN means:
> > >
> > > defined(TARGET_WORDS_BIGENDIAN) ? DEVICE_BIG_ENDIAN :
> > > DEVICE_NATIVE_ENDIAN
> > >
> > > i.e. the device has the same endianness as the guest cpu.
> >
> > I think this boils down to the same thing in the end, no?
>
> Maybe.
>
> > However, it's a bad way to describe the setup
> > for device writers: it documents the
> > internal workings of qemu with multiple
> > swaps. We need to document the end result.
> >
> > And, it is IMO confusing to say that 'a device expects data'
> > this adds a speculative element where you
> > are asked to think about what you would want to
> > do and promised that this will be somehow
> > satisfied.
> >
> > Instead, please specify what the API does, users
> > can make their own decisions on when to use it.
>
> But "callbacks accept data in little endian format" implies that you
> have to add a swap in the handler,
> since you usually want data in host endian.
> 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.
+
+ 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.
+
+ 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.
+ 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.
+
