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Re: [PATCH for-7.0 v2 1/2] ivshmem.c: change endianness to LITTLE_ENDIAN


From: Daniel Henrique Barboza
Subject: Re: [PATCH for-7.0 v2 1/2] ivshmem.c: change endianness to LITTLE_ENDIAN
Date: Wed, 24 Nov 2021 08:33:31 -0300
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.2.0



On 11/24/21 08:28, Philippe Mathieu-Daudé wrote:
On 11/24/21 10:29, Daniel Henrique Barboza wrote:
The ivshmem device, as with most PCI devices, uses little endian byte
order. However, the endianness of its mmio_ops is marked as
DEVICE_NATIVE_ENDIAN. This presents not only the usual problems with big
endian hosts but also with PowerPC little endian hosts as well, since
the Power architecture in QEMU uses big endian hardware (XIVE controller,
PCI Host Bridges, etc) even if the host is in little endian byte order.

Maybe mention commit f7a199b2b44 ("ivshmem: use little-endian
int64_t for the protocol")?


Good point. I'll send a v3.


As it is today, the IVPosition of the device will be byte swapped when
running in Power BE and LE. This can be seen by changing the existing
qtest 'ivshmem-test' to run in ppc64 hosts and printing the IVPOSITION
regs in test_ivshmem_server() right after the VM ids assert. For x86_64
the VM id values read are '0' and '1', for ppc64 (tested in a Power8
RHEL 7.9 BE server) and ppc64le (tested in a Power9 RHEL 8.6 LE server)
the ids will be '0' and '0x1000000'.

Change this device to LITTLE_ENDIAN fixes the issue for Power hosts of
both endianness, and every other big-endian architecture that might use
this device, without impacting x86 users.

Fixes: cb06608e17f8 ("ivshmem: convert to memory API")
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/168
Signed-off-by: Daniel Henrique Barboza <danielhb413@gmail.com>
---
  hw/misc/ivshmem.c | 2 +-
  1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/hw/misc/ivshmem.c b/hw/misc/ivshmem.c
index 1ba4a98377..299837e5c1 100644
--- a/hw/misc/ivshmem.c
+++ b/hw/misc/ivshmem.c
@@ -243,7 +243,7 @@ static uint64_t ivshmem_io_read(void *opaque, hwaddr addr,
  static const MemoryRegionOps ivshmem_mmio_ops = {
      .read = ivshmem_io_read,
      .write = ivshmem_io_write,
-    .endianness = DEVICE_NATIVE_ENDIAN,
+    .endianness = DEVICE_LITTLE_ENDIAN,
      .impl = {
          .min_access_size = 4,
          .max_access_size = 4,


Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>


Thanks!


Daniel





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