Re: [Qemu-devel] VFIO NATIVE_ENDIAN regions question

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Sat,  6 Sep 2014 12:49:47 +1000
Alexey Kardashevskiy <aik@ozlabs.ru> wrote:

> 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)

AFAIK these accessors implement target CPU accesses to memory... I
don't see why they should be affected by the host endianness.
If you have an endianness issue related to HOST_WORDS_BIGENDIAN,
my guess is that you need to cancel the byteswap in hw/misc/vfio.c.

>  #define lduw_p(p) lduw_be_p(p)
>  #define ldsw_p(p) ldsw_be_p(p)
>  #define ldl_p(p) ldl_be_p(p)

-- 
Gregory Kurz                                     kurzgreg@fr.ibm.com
                                                 gkurz@linux.vnet.ibm.com
Software Engineer @ IBM/Meiosys                  http://www.ibm.com
Tel +33 (0)562 165 496

"Anarchy is about taking complete responsibility for yourself."
        Alan Moore.