Re: [PATCH] hw/virtio: Update vring after modifying its queue size

["Philippe Mathieu-Daudé" <philmd@redhat.com>]

On 8/26/21 12:42 AM, Philippe Mathieu-Daudé wrote:
> When a ring queue size is modified, we need to call
> virtio_queue_update_rings() to re-init the memory region
> caches. Otherwise the region might have outdated memory
> size, and later load/store access might trigger an
> assertion, such:
> 
>   qemu-system-i386: include/exec/memory_ldst_cached.h.inc:30: uint16_t address_space_lduw_le_cached(MemoryRegionCache *, hwaddr, MemTxAttrs, MemTxResult *):
>   Assertion `addr < cache->len && 2 <= cache->len - addr' failed.
>   Thread 1 "qemu-system-i38" received signal SIGABRT, Aborted.
>   0x00007ffff4d312a2 in raise () from /lib64/libc.so.6
>   (gdb) bt
>   #1  0x00007ffff4d1a8a4 in abort () at /lib64/libc.so.6
>   #4  0x0000555558f2b8ec in address_space_lduw_le_cached (cache=0x61300010a7c0, addr=134, attrs=..., result=0x0) at include/exec/memory_ldst_cached.h.inc:30
>   #5  0x0000555558f2ac6b in lduw_le_phys_cached (cache=0x61300010a7c0, addr=134) at include/exec/memory_ldst_phys.h.inc:67
>   #6  0x0000555558f2a3cd in virtio_lduw_phys_cached (vdev=0x62d00003a680, cache=0x61300010a7c0, pa=134) at include/hw/virtio/virtio-access.h:166
>   #7  0x0000555558f300ea in vring_avail_ring (vq=0x7fffdd55d8a0, i=65) at hw/virtio/virtio.c:326
>   #8  0x0000555558f33b10 in vring_get_used_event (vq=0x7fffdd55d8a0) at hw/virtio/virtio.c:332
>   #9  0x0000555558f33677 in virtio_split_should_notify (vdev=0x62d00003a680, vq=0x7fffdd55d8a0) at hw/virtio/virtio.c:2471
>   #10 0x0000555558f1859f in virtio_should_notify (vdev=0x62d00003a680, vq=0x7fffdd55d8a0) at hw/virtio/virtio.c:2523
>   #11 0x0000555558f188cc in virtio_notify (vdev=0x62d00003a680, vq=0x7fffdd55d8a0) at hw/virtio/virtio.c:2565
>   #12 0x0000555557c2bd52 in virtio_input_handle_sts (vdev=0x62d00003a680, vq=0x7fffdd55d8a0) at hw/input/virtio-input.c:100
>   #13 0x0000555558f16df7 in virtio_queue_notify (vdev=0x62d00003a680, n=1) at hw/virtio/virtio.c:2363
>   #14 0x00005555583f45c0 in virtio_pci_notify_write (opaque=0x62d000032400, addr=7, val=0, size=1) at hw/virtio/virtio-pci.c:1369
>   #15 0x0000555558b80b04 in memory_region_write_accessor (mr=0x62d000033190, addr=7, value=0x7fffffff8eb0, size=1, shift=0, mask=255, attrs=...) at softmmu/memory.c:492

Earlier call which updated vring.num:

memory_region_ops_write cpu -1 mr 0x62d000032e60 addr 0xe0004018 value
0x41 size 1 name 'virtio-pci-common-virtio-input'
Thread 1 "qemu-system-i38" hit Breakpoint 1, virtio_queue_set_num
(vdev=0x62d00003a680, n=1, num=65) at hw/virtio/virtio.c:2276
2276        if (!!num != !!vdev->vq[n].vring.num ||
(gdb) bt
#0  virtio_queue_set_num (vdev=0x62d00003a680, n=1, num=65) at
hw/virtio/virtio.c:2276
#1  0x00005555583f2a2c in virtio_pci_common_write
(opaque=0x62d000032400, addr=24, val=65, size=1) at
hw/virtio/virtio-pci.c:1297
#2  0x0000555558b81b04 in memory_region_write_accessor
(mr=0x62d000032e60, addr=24, value=0x7fffffff8eb0, size=1, shift=0,
mask=255, attrs=...) at softmmu/memory.c:492
#3  0x0000555558b81442 in access_with_adjusted_size (addr=24,
value=0x7fffffff8eb0, size=1, access_size_min=1, access_size_max=4,
access_fn=
    0x555558b81500 <memory_region_write_accessor>, mr=0x62d000032e60,
attrs=...) at softmmu/memory.c:554
#4  0x0000555558b7fd57 in memory_region_dispatch_write
(mr=0x62d000032e60, addr=24, data=65, op=MO_8, attrs=...) at
softmmu/memory.c:1504
#5  0x0000555558b513f4 in flatview_write_continue (fv=0x60600005cc00,
addr=3758112792, attrs=..., ptr=0x6020000dbb90, len=1, addr1=24, l=1,
mr=0x62d000032e60)
    at softmmu/physmem.c:2777
#6  0x0000555558b3fdd0 in flatview_write (fv=0x60600005cc00,
addr=3758112792, attrs=..., buf=0x6020000dbb90, len=1) at
softmmu/physmem.c:2817
#7  0x0000555558b3f959 in address_space_write (as=0x60800000d920,
addr=3758112792, attrs=..., buf=0x6020000dbb90, len=1) at
softmmu/physmem.c:2909

> Reported-by: Qiuhao Li <Qiuhao.Li@outlook.com>
> Fixes: ab223c9518e ("virtio: allow virtio-1 queue layout")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/302

BTW this also fixes #300 and #301 :)

> BugLink: https://bugs.launchpad.net/qemu/+bug/1913510
> Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
> ---
>  hw/virtio/virtio.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 874377f37a7..04ffe5f420e 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -2255,6 +2255,7 @@ void virtio_queue_set_num(VirtIODevice *vdev, int n, int num)
>          return;
>      }
>      vdev->vq[n].vring.num = num;
> +    virtio_queue_update_rings(vdev, n);
>  }
>  
>  VirtQueue *virtio_vector_first_queue(VirtIODevice *vdev, uint16_t vector)
>