[Qemu-devel] VFIO NATIVE_ENDIAN regions question

[Alexey Kardashevskiy]

At the moment VFIO's BARs are NATIVE_ENDIAN. The idea is that since
it does not parse BARs content and just provides transport, it should
not do byte swaps, the guest does it anyway. That worked fine while
the host was big-endian and it does not work when the host is little-endian.
This happens because ./configure defines static macro TARGET_WORDS_BIGENDIAN
for ppc64 and since there is no ppc64le - endianness of host and guest does
not match and stl_p&co swaps bytes as shown below.

The proposed patch at the end of this mail fixes the issue for VFIO in
any combination of host-guest, BE-LE (need to double check with BE host).
However since I fail to grasp the idea of having statically defined
TARGET_WORDS_BIGENDIAN, I assume I am missing something big here.

What would the correct fix be here? Thanks!



Breakpoint 9, vfio_bar_read (opaque=0x10de2e98, addr=0x6f4, size=0x4) at 
/home/alexey/p/qemu/hw/misc/vfio.c:1140
1140    {
Missing separate debuginfos, use: zypper install 
glibc-debuginfo-2.19-15.1.ppc64le 
libgcc_s1-debuginfo-4.8.3+r212056-4.17.ppc64le 
libglib-2_0-0-debuginfo-2.38.2-5.8.ppc64le libgthread-2_0-0-debu
ginfo-2.38.2-5.8.ppc64le libpcre1-debuginfo-8.33-3.253.ppc64le 
libpixman-1-0-debuginfo-0.32.6-1.1.ppc64le 
libstdc++6-debuginfo-4.8.3+r212056-4.17.ppc64le 
libz1-debuginfo-1.2.8-4.65.ppc64le
(gdb) bt
#0  vfio_bar_read (opaque=0x10de2e98, addr=0x6f4, size=0x4) at 
/home/alexey/p/qemu/hw/misc/vfio.c:1140
#1  0x00000000100a4bd4 in memory_region_read_accessor (mr=0x10de2ea8, 
addr=0x6f4, value=0x3fffb77de320, size=0x4, shift=0x0, mask=0xffffffff) at 
/home/alexey/p/qemu/memory.c:410
#2  0x00000000100a5000 in access_with_adjusted_size (addr=0x6f4, 
value=0x3fffb77de320, size=0x4, access_size_min=0x1, access_size_max=0x4, 
access=0x100a4b38 <memory_region_read_accessor>, mr=0x1
0de2ea8) at /home/alexey/p/qemu/memory.c:475
#3  0x00000000100a84ac in memory_region_dispatch_read1 (mr=0x10de2ea8, 
addr=0x6f4, size=0x4) at /home/alexey/p/qemu/memory.c:1097
#4  0x00000000100a85d8 in memory_region_dispatch_read (mr=0x10de2ea8, 
addr=0x6f4, pval=0x3fffb77de458, size=0x4) at /home/alexey/p/qemu/memory.c:1119
#5  0x00000000100ace24 in io_mem_read (mr=0x10de2ea8, addr=0x6f4, 
pval=0x3fffb77de458, size=0x4) at /home/alexey/p/qemu/memory.c:1967
#6  0x0000000010013bf8 in address_space_rw (as=0x1081d410 
<address_space_memory>, addr=0x1a0b00006f4, buf=0x3fffb7f80028 "", len=0x4, 
is_write=0x0) at /home/alexey/p/qemu/exec.c:2076
#7  0x0000000010013fa4 in cpu_physical_memory_rw (addr=0x1a0b00006f4, 
buf=0x3fffb7f80028 "", len=0x4, is_write=0x0) at /home/alexey/p/qemu/exec.c:2121
#8  0x00000000100a015c in kvm_cpu_exec (cpu=0x3fffb77e0010) at 
/home/alexey/p/qemu/kvm-all.c:1770
#9  0x000000001007ab18 in qemu_kvm_cpu_thread_fn (arg=0x3fffb77e0010) at 
/home/alexey/p/qemu/cpus.c:940
#10 0x00003fffb7828a64 in start_thread () from /lib64/libpthread.so.0
#11 0x00003fffb7993b00 in clone () from /lib64/libc.so.6
(gdb) n
1141        VFIOBAR *bar = opaque;
(gdb)
1148        uint64_t data = 0;
(gdb)
1150        if (pread(bar->fd, &buf, size, bar->fd_offset + addr) != size) {
(gdb)
1156        switch (size) {
(gdb)
1164            data = buf.dword;
(gdb)
1165            break;
(gdb)
1171        if (addr == 0x6f4) {
(gdb)
1172            printf("%s %u size=%d val=%lx\n", __func__, __LINE__, size, 
data);
(gdb)
vfio_bar_read 1172 size=4 val=4
1187        vfio_eoi(container_of(bar, VFIODevice, bars[bar->nr]));
(gdb)
1189        return data;
(gdb)
1190    }
(gdb)
memory_region_read_accessor (mr=0x10de2ea8, addr=0x6f4, value=0x3fffb77de320, 
size=0x4, shift=0x0, mask=0xffffffff) at /home/alexey/p/qemu/memory.c:411
411         trace_memory_region_ops_read(mr, addr, tmp, size);
(gdb)
412         *value |= (tmp & mask) << shift;
(gdb)
413     }
(gdb)
access_with_adjusted_size (addr=0x6f4, value=0x3fffb77de320, size=0x4, 
access_size_min=0x1, access_size_max=0x4, access=0x100a4b38 
<memory_region_read_accessor>, mr=0x10de2ea8) at /home/alexey/p
/qemu/memory.c:474
474             for (i = 0; i < size; i += access_size) {
(gdb)
483     }
(gdb)
memory_region_dispatch_read1 (mr=0x10de2ea8, addr=0x6f4, size=0x4) at 
/home/alexey/p/qemu/memory.c:1106
1106        return data;
(gdb)
1107    }
(gdb)
memory_region_dispatch_read (mr=0x10de2ea8, addr=0x6f4, pval=0x3fffb77de458, 
size=0x4) at /home/alexey/p/qemu/memory.c:1120
1120        adjust_endianness(mr, pval, size);
(gdb) s
adjust_endianness (mr=0x10de2ea8, data=0x3fffb77de458, size=0x4) at 
/home/alexey/p/qemu/memory.c:364
364     {
(gdb) n
365         if (memory_region_wrong_endianness(mr)) {
(gdb)
382     }
(gdb)
memory_region_dispatch_read (mr=0x10de2ea8, addr=0x6f4, pval=0x3fffb77de458, 
size=0x4) at /home/alexey/p/qemu/memory.c:1121
1121        return false;
(gdb)
1122    }
(gdb)
io_mem_read (mr=0x10de2ea8, addr=0x6f4, pval=0x3fffb77de458, size=0x4) at 
/home/alexey/p/qemu/memory.c:1968
1968    }
(gdb)
address_space_rw (as=0x1081d410 <address_space_memory>, addr=0x1a0b00006f4, 
buf=0x3fffb7f80028 "", len=0x4, is_write=0x0) at /home/alexey/p/qemu/exec.c:2077
2077                        stl_p(buf, val);
(gdb) s
stl_be_p (ptr=0x3fffb7f80028, v=0x4) at 
/home/alexey/p/qemu/include/qemu/bswap.h:377
377     {
(gdb) n

---
 include/exec/cpu-all.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
index f9d132f..f3a761a 100644
--- a/include/exec/cpu-all.h
+++ b/include/exec/cpu-all.h
@@ -152,7 +152,7 @@ static inline void tswap64s(uint64_t *s)
  */
 
 /* target-endianness CPU memory access functions */
-#if defined(TARGET_WORDS_BIGENDIAN)
+#if defined(HOST_WORDS_BIGENDIAN)
 #define lduw_p(p) lduw_be_p(p)
 #define ldsw_p(p) ldsw_be_p(p)
 #define ldl_p(p) ldl_be_p(p)
-- 
2.0.0