When performing the following command in a Xen VM the call trace below is
generated.

    sudo fstrim -v /

Xen VM host: Xen 4.14.0
Xen Dom0: Linux 4.19.14
Xen DomX: Linux 5.10.6

The code in the kernel triggering this trace is the following.

        /* In case the discard granularity isn't set by buggy device driver
*/
        if (WARN_ON_ONCE(!q->limits.discard_granularity)) {
                char dev_name[BDEVNAME_SIZE];

                bdevname(bdev, dev_name);
                pr_err_ratelimited("%s: Error: discard_granularity is
0.\n", dev_name);
                return -EOPNOTSUPP;
        }

The call trace in the Xen VM.

[  145.295257] ------------[ cut here ]------------
[  145.295274] WARNING: CPU: 1 PID: 1230 at block/blk-lib.c:51
__blkdev_issue_discard+0x245/0x2a0
[  145.295277] Modules linked in: intel_rapl_msr intel_rapl_common
crct10dif_pclmul crc32_pclmul ghash_clmulni_intel aesni_intel crypto_simd
cryptd glue_helper rapl joydev bochs_drm drm_vram_helper drm_ttm_helper
mousedev xen_kbdfront ttm drm_kms_helper cec syscopyarea sysfillrect
sysimgblt xen_netfront fb_sys_fops intel_agp pcspkr intel_gtt mac_hid fuse
drm agpgart bpf_preload ip_tables x_tables ext4 crc32c_generic crc16
mbcache jbd2 ata_generic pata_acpi floppy xen_blkfront crc32c_intel
serio_raw ata_piix
[  145.295373] CPU: 1 PID: 1230 Comm: fstrim Not tainted 5.10.6-arch1-1 #1
[  145.295376] Hardware name: Xen HVM domU, BIOS 4.14.0 11/14/2020
[  145.295383] RIP: 0010:__blkdev_issue_discard+0x245/0x2a0
[  145.295390] Code: 48 8b 44 24 48 65 48 2b 04 25 28 00 00 00 75 6c 8b 44
24 1c 48 83 c4 50 5b 5d 41 5c 41 5d 41 5e 41 5f c3 0f 0b e9 f9 fe ff ff
<0f> 0b 48 8d 74 24 28 4c 89 ef e8 ac c2 00 00 48 c7 c6 e0 a1 86 b6
[  145.295393] RSP: 0018:ffffb283007b7b70 EFLAGS: 00010246
[  145.295399] RAX: 0000000000000000 RBX: 0000000000000000 RCX:
0000000000000c40
[  145.295402] RDX: 00000000000000c0 RSI: 0000000000012240 RDI:
ffff8e4980411380
[  145.295404] RBP: 00000000000000c0 R08: 0000000000000000 R09:
ffffb283007b7bf8
[  145.295407] R10: 0000000000000001 R11: 0000000000002000 R12:
0000000000012240
[  145.295410] R13: ffff8e4980411380 R14: ffff8e4983b14770 R15:
0000000000000000
[  145.295415] FS:  00007fd5189fc580(0000) GS:ffff8e498ad00000(0000)
knlGS:0000000000000000
[  145.295418] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[  145.295421] CR2: 00005562e06d6448 CR3: 0000000108188001 CR4:
00000000003706e0
[  145.295432] DR0: 0000000000000000 DR1: 0000000000000000 DR2:
0000000000000000
[  145.295435] DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7:
0000000000000400
[  145.295437] Call Trace:
[  145.295452]  blkdev_issue_discard+0x86/0xe0
[  145.295510]  ext4_trim_fs+0x518/0x930 [ext4]
[  145.295521]  ? mntput_no_expire+0x4a/0x260
[  145.295568]  __ext4_ioctl+0xfee/0x1b10 [ext4]
[  145.295619]  ext4_ioctl+0x2a/0x40 [ext4]
[  145.295626]  __x64_sys_ioctl+0x83/0xb0
[  145.295634]  do_syscall_64+0x33/0x40
[  145.295640]  entry_SYSCALL_64_after_hwframe+0x44/0xa9
[  145.295646] RIP: 0033:0x7fd518b71f6b
[  145.295652] Code: 89 d8 49 8d 3c 1c 48 f7 d8 49 39 c4 72 b5 e8 1c ff ff
ff 85 c0 78 ba 4c 89 e0 5b 5d 41 5c c3 f3 0f 1e fa b8 10 00 00 00 0f 05
<48> 3d 01 f0 ff ff 73 01 c3 48 8b 0d d5 ae 0c 00 f7 d8 64 89 01 48
[  145.295655] RSP: 002b:00007ffea645a438 EFLAGS: 00000246 ORIG_RAX:
0000000000000010
[  145.295660] RAX: ffffffffffffffda RBX: 00007ffea645a510 RCX:
00007fd518b71f6b
[  145.295662] RDX: 00007ffea645a450 RSI: 00000000c0185879 RDI:
0000000000000003
[  145.295665] RBP: 00005562e06d5440 R08: 00005562e06d5440 R09:
00007ffea645ae87
[  145.295668] R10: 0000000000000000 R11: 0000000000000246 R12:
0000000000000003
[  145.295670] R13: 00007ffea645ae86 R14: 0000000000000000 R15:
ffffffff00000000
[  145.295677] ---[ end trace 5fcef9628995f731 ]---
[  145.295682] xvda1: Error: discard_granularity is 0.
[  145.295703] xvda1: Error: discard_granularity is 0.

Please note that the VM is using a RAW partition as a disk on the VM host.

    <disk type='file' device='disk'>
      <driver name='qemu' type='raw'/>
      <source file='/dev/nvme0n1p2'/>
      <target dev='xvda' bus='xen'/>
    </disk>

Attachments: xen-dmesg, dom0-dmesg, domx-dmesg, domx-libvirt-xml are
attached.

-- 
Arthur Borsboom