On 2/28/25 8:20 PM, Steven Sistare wrote:
> On 2/28/2025 1:13 PM, Steven Sistare wrote:
>> On 2/28/2025 12:39 PM, Andrey Drobyshev wrote:
>>> Hi all,
>>>
>>> We've been experimenting with cpr-transfer migration mode recently and
>>> have discovered the following issue with the guest QXL driver:
>>>
>>> Run migration source:
>>>> EMULATOR=/path/to/emulator
>>>> ROOTFS=/path/to/image
>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>>>>
>>>> $EMULATOR -enable-kvm \
>>>> -machine q35 \
>>>> -cpu host -smp 2 -m 2G \
>>>> -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>>>> ram0,share=on\
>>>> -machine memory-backend=ram0 \
>>>> -machine aux-ram-share=on \
>>>> -drive file=$ROOTFS,media=disk,if=virtio \
>>>> -qmp unix:$QMPSOCK,server=on,wait=off \
>>>> -nographic \
>>>> -device qxl-vga
>>>
>>> Run migration target:
>>>> EMULATOR=/path/to/emulator
>>>> ROOTFS=/path/to/image
>>>> QMPSOCK=/var/run/alma8qmp-dst.sock
>>>> $EMULATOR -enable-kvm \
>>>> -machine q35 \
>>>> -cpu host -smp 2 -m 2G \
>>>> -object memory-backend-file,id=ram0,size=2G,mem-path=/dev/shm/
>>>> ram0,share=on\
>>>> -machine memory-backend=ram0 \
>>>> -machine aux-ram-share=on \
>>>> -drive file=$ROOTFS,media=disk,if=virtio \
>>>> -qmp unix:$QMPSOCK,server=on,wait=off \
>>>> -nographic \
>>>> -device qxl-vga \
>>>> -incoming tcp:0:44444 \
>>>> -incoming '{"channel-type": "cpr", "addr": { "transport":
>>>> "socket", "type": "unix", "path": "/var/run/alma8cpr-dst.sock"}}'
>>>
>>>
>>> Launch the migration:
>>>> QMPSHELL=/root/src/qemu/master/scripts/qmp/qmp-shell
>>>> QMPSOCK=/var/run/alma8qmp-src.sock
>>>>
>>>> $QMPSHELL -p $QMPSOCK <<EOF
>>>> migrate-set-parameters mode=cpr-transfer
>>>> migrate channels=[{"channel-type":"main","addr":
>>>> {"transport":"socket","type":"inet","host":"0","port":"44444"}},
>>>> {"channel-type":"cpr","addr":
>>>> {"transport":"socket","type":"unix","path":"/var/run/alma8cpr-
>>>> dst.sock"}}]
>>>> EOF
>>>
>>> Then, after a while, QXL guest driver on target crashes spewing the
>>> following messages:
>>>> [ 73.962002] [TTM] Buffer eviction failed
>>>> [ 73.962072] qxl 0000:00:02.0: object_init failed for (3149824,
>>>> 0x00000001)
>>>> [ 73.962081] [drm:qxl_alloc_bo_reserved [qxl]] *ERROR* failed to
>>>> allocate VRAM BO
>>>
>>> That seems to be a known kernel QXL driver bug:
>>>
>>> https://lore.kernel.org/all/20220907094423.93581-1-min_h...@163.com/T/
>>> https://lore.kernel.org/lkml/ztgydqrlk6wx_...@eldamar.lan/
>>>
>>> (the latter discussion contains that reproduce script which speeds up
>>> the crash in the guest):
>>>> #!/bin/bash
>>>>
>>>> chvt 3
>>>>
>>>> for j in $(seq 80); do
>>>> echo "$(date) starting round $j"
>>>> if [ "$(journalctl --boot | grep "failed to allocate VRAM
>>>> BO")" != "" ]; then
>>>> echo "bug was reproduced after $j tries"
>>>> exit 1
>>>> fi
>>>> for i in $(seq 100); do
>>>> dmesg > /dev/tty3
>>>> done
>>>> done
>>>>
>>>> echo "bug could not be reproduced"
>>>> exit 0
>>>
>>> The bug itself seems to remain unfixed, as I was able to reproduce that
>>> with Fedora 41 guest, as well as AlmaLinux 8 guest. However our
>>> cpr-transfer code also seems to be buggy as it triggers the crash -
>>> without the cpr-transfer migration the above reproduce doesn't lead to
>>> crash on the source VM.
>>>
>>> I suspect that, as cpr-transfer doesn't migrate the guest memory, but
>>> rather passes it through the memory backend object, our code might
>>> somehow corrupt the VRAM. However, I wasn't able to trace the
>>> corruption so far.
>>>
>>> Could somebody help the investigation and take a look into this? Any
>>> suggestions would be appreciated. Thanks!
>>
>> Possibly some memory region created by qxl is not being preserved.
>> Try adding these traces to see what is preserved:
>>
>> -trace enable='*cpr*'
>> -trace enable='*ram_alloc*'
>
> Also try adding this patch to see if it flags any ram blocks as not
> compatible with cpr. A message is printed at migration start time.
> https://lore.kernel.org/qemu-devel/1740667681-257312-1-git-send-email-
> steven.sist...@oracle.com/
>
> - Steve
>
With the traces enabled + the "migration: ram block cpr blockers" patch
applied:
Source:
> cpr_find_fd pc.bios, id 0 returns -1
> cpr_save_fd pc.bios, id 0, fd 22
> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 22 host
> 0x7fec18e00000
> cpr_find_fd pc.rom, id 0 returns -1
> cpr_save_fd pc.rom, id 0, fd 23
> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 23 host
> 0x7fec18c00000
> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns -1
> cpr_save_fd 0000:00:01.0/e1000e.rom, id 0, fd 24
> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
> 24 host 0x7fec18a00000
> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns -1
> cpr_save_fd 0000:00:02.0/vga.vram, id 0, fd 25
> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
> fd 25 host 0x7feb77e00000
> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns -1
> cpr_save_fd 0000:00:02.0/qxl.vrom, id 0, fd 27
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 27
> host 0x7fec18800000
> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns -1
> cpr_save_fd 0000:00:02.0/qxl.vram, id 0, fd 28
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
> fd 28 host 0x7feb73c00000
> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns -1
> cpr_save_fd 0000:00:02.0/qxl.rom, id 0, fd 34
> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 34
> host 0x7fec18600000
> cpr_find_fd /rom@etc/acpi/tables, id 0 returns -1
> cpr_save_fd /rom@etc/acpi/tables, id 0, fd 35
> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 35
> host 0x7fec18200000
> cpr_find_fd /rom@etc/table-loader, id 0 returns -1
> cpr_save_fd /rom@etc/table-loader, id 0, fd 36
> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 36
> host 0x7feb8b600000
> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns -1
> cpr_save_fd /rom@etc/acpi/rsdp, id 0, fd 37
> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 37 host
> 0x7feb8b400000
>
> cpr_state_save cpr-transfer mode
> cpr_transfer_output /var/run/alma8cpr-dst.sock
Target:
> cpr_transfer_input /var/run/alma8cpr-dst.sock
> cpr_state_load cpr-transfer mode
> cpr_find_fd pc.bios, id 0 returns 20
> qemu_ram_alloc_shared pc.bios size 262144 max_size 262144 fd 20 host
> 0x7fcdc9800000
> cpr_find_fd pc.rom, id 0 returns 19
> qemu_ram_alloc_shared pc.rom size 131072 max_size 131072 fd 19 host
> 0x7fcdc9600000
> cpr_find_fd 0000:00:01.0/e1000e.rom, id 0 returns 18
> qemu_ram_alloc_shared 0000:00:01.0/e1000e.rom size 262144 max_size 262144 fd
> 18 host 0x7fcdc9400000
> cpr_find_fd 0000:00:02.0/vga.vram, id 0 returns 17
> qemu_ram_alloc_shared 0000:00:02.0/vga.vram size 67108864 max_size 67108864
> fd 17 host 0x7fcd27e00000
> cpr_find_fd 0000:00:02.0/qxl.vrom, id 0 returns 16
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vrom size 8192 max_size 8192 fd 16
> host 0x7fcdc9200000
> cpr_find_fd 0000:00:02.0/qxl.vram, id 0 returns 15
> qemu_ram_alloc_shared 0000:00:02.0/qxl.vram size 67108864 max_size 67108864
> fd 15 host 0x7fcd23c00000
> cpr_find_fd 0000:00:02.0/qxl.rom, id 0 returns 14
> qemu_ram_alloc_shared 0000:00:02.0/qxl.rom size 65536 max_size 65536 fd 14
> host 0x7fcdc8800000
> cpr_find_fd /rom@etc/acpi/tables, id 0 returns 13
> qemu_ram_alloc_shared /rom@etc/acpi/tables size 131072 max_size 2097152 fd 13
> host 0x7fcdc8400000
> cpr_find_fd /rom@etc/table-loader, id 0 returns 11
> qemu_ram_alloc_shared /rom@etc/table-loader size 4096 max_size 65536 fd 11
> host 0x7fcdc8200000
> cpr_find_fd /rom@etc/acpi/rsdp, id 0 returns 10
> qemu_ram_alloc_shared /rom@etc/acpi/rsdp size 4096 max_size 4096 fd 10 host
> 0x7fcd3be00000
Looks like both vga.vram and qxl.vram are being preserved (with the same
addresses), and no incompatible ram blocks are found during migration.
Andrey
