[Qemu-devel] [PATCH for-2.0] hw/pci-host/prep: Don't reverse IO accesses on bigendian hosts

[Qemu-devel] [PATCH for-2.0] hw/pci-host/prep: Don't reverse IO accesses on bigendian hosts

[Peter Maydell]

The raven_io_read() and raven_io_write() functions pass and
return values in little-endian format (since the IO op struct
is marked DEVICE_LITTLE_ENDIAN); however they were storing the
values in the buffer to pass to address_space_read/write()
in host-endian order, which meant that on big-endian hosts
the values were inadvertently reversed. Use the *_le_p()
accessors instead so that we are consistent regardless of
host endianness.

Strictly speaking the byte order of the buffer for
address_space_rw() is target byte order (which for PPC
will be BE) but it doesn't actually matter as long as we
are consistent about the marking on the IO op struct and
which stl_*_p().

This bug was probably introduced due to confusion caused by
the two different versions of ldl_p() and friends:
 bswap.h defines versions meaning "host endianness access"
 cpu-all.h defines versions meaning "target endianness access"
As a target-independent source file prep.c gets the bswap.h
versions; the very similar looking code in ioport.c is
compiled per-target and gets the cpu-all.h versions.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
"Why is a raven like a writing desk?
 Because it is nevar put with the wrong end in front!"
   -- Lewis Carroll

This fixes the endianness test failure on bigendian hosts.
HOWEVER I have not actually tested it with a guest :-)
and endianness issues are notoriously hard to reason about
correctly. Review appreciated.

RTH suggests that we rename the cpu-all.h ldl_p &c to
ldl_te_p() &c (for 'target endianness') to reduce confusion;
I agree but this probably also requires some auditing of
users to check for other mistaken uses and in any case is
2.1 material.

 hw/pci-host/prep.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/hw/pci-host/prep.c b/hw/pci-host/prep.c
index d3e746c..4014540 100644
--- a/hw/pci-host/prep.c
+++ b/hw/pci-host/prep.c
@@ -145,9 +145,9 @@ static uint64_t raven_io_read(void *opaque, hwaddr addr,
     if (size == 1) {
         return buf[0];
     } else if (size == 2) {
-        return lduw_p(buf);
+        return lduw_le_p(buf);
     } else if (size == 4) {
-        return ldl_p(buf);
+        return ldl_le_p(buf);
     } else {
         g_assert_not_reached();
     }
@@ -164,9 +164,9 @@ static void raven_io_write(void *opaque, hwaddr addr,
     if (size == 1) {
         buf[0] = val;
     } else if (size == 2) {
-        stw_p(buf, val);
+        stw_le_p(buf, val);
     } else if (size == 4) {
-        stl_p(buf, val);
+        stl_le_p(buf, val);
     } else {
         g_assert_not_reached();
     }
-- 
1.9.1

Re: [Qemu-devel] [PATCH for-2.0] hw/pci-host/prep: Don't reverse IO accesses on bigendian hosts

[Peter Maydell]

On 8 April 2014 16:51, Peter Maydell <peter.maydell@linaro.org> wrote:
> This fixes the endianness test failure on bigendian hosts.
> HOWEVER I have not actually tested it with a guest :-)

Alex found me a prep kernel. It seems to have issues with
its userspace, but; without this patch, when we probe the
IDE devices we get:

hdc: EQUMD DVR-MO, ATAPI cdrom or floppy?, assuming FLOPPY drive

(spot the byte-reversed halfwords!)

and with the patch we get:

hdc: QEMU DVD-ROM, ATAPI CD/DVD-ROM drive

so I'm pretty confident this patch is not just fixing a
test case but also fixing problems with real guests.

thanks
-- PMM

Re: [Qemu-devel] [PATCH for-2.0] hw/pci-host/prep: Don't reverse IO accesses on bigendian hosts

[Richard Henderson]

On 04/08/2014 08:51 AM, Peter Maydell wrote:
> The raven_io_read() and raven_io_write() functions pass and
> return values in little-endian format (since the IO op struct
> is marked DEVICE_LITTLE_ENDIAN); however they were storing the
> values in the buffer to pass to address_space_read/write()
> in host-endian order, which meant that on big-endian hosts
> the values were inadvertently reversed. Use the *_le_p()
> accessors instead so that we are consistent regardless of
> host endianness.
> 
> Strictly speaking the byte order of the buffer for
> address_space_rw() is target byte order (which for PPC
> will be BE) but it doesn't actually matter as long as we
> are consistent about the marking on the IO op struct and
> which stl_*_p().
> 
> This bug was probably introduced due to confusion caused by
> the two different versions of ldl_p() and friends:
>  bswap.h defines versions meaning "host endianness access"
>  cpu-all.h defines versions meaning "target endianness access"
> As a target-independent source file prep.c gets the bswap.h
> versions; the very similar looking code in ioport.c is
> compiled per-target and gets the cpu-all.h versions.
> 
> Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> ---
> "Why is a raven like a writing desk?
>  Because it is nevar put with the wrong end in front!"
>    -- Lewis Carroll

Ha ha.

> 
> This fixes the endianness test failure on bigendian hosts.
> HOWEVER I have not actually tested it with a guest :-)
> and endianness issues are notoriously hard to reason about
> correctly. Review appreciated.
> 
> RTH suggests that we rename the cpu-all.h ldl_p &c to
> ldl_te_p() &c (for 'target endianness') to reduce confusion;
> I agree but this probably also requires some auditing of
> users to check for other mistaken uses and in any case is
> 2.1 material.
> 
>  hw/pci-host/prep.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)

Reviewed-by: Richard Henderson <rth@twiddle.net>


r~

Re: [Qemu-devel] [PATCH for-2.0] hw/pci-host/prep: Don't reverse IO accesses on bigendian hosts

[Peter Maydell]

On 8 April 2014 18:24, Richard Henderson <rth@twiddle.net> wrote:
> On 04/08/2014 08:51 AM, Peter Maydell wrote:
>> The raven_io_read() and raven_io_write() functions pass and
>> return values in little-endian format (since the IO op struct
>> is marked DEVICE_LITTLE_ENDIAN); however they were storing the
>> values in the buffer to pass to address_space_read/write()
>> in host-endian order, which meant that on big-endian hosts
>> the values were inadvertently reversed. Use the *_le_p()
>> accessors instead so that we are consistent regardless of
>> host endianness.

> Reviewed-by: Richard Henderson <rth@twiddle.net>

Thanks; applied to master.

-- PMM