[Qemu-devel] The WCE issue in guest when i enable WCE in Qemu side.

[Zhong, Yang]

Hello all,

I am using the Qemu/SPDK to do vhost-user-blk verification, and the qemu 
command as below:

./qemu-system-x86_64 \
    -machine q35,accel=kvm \
    -cpu host -m 1024,maxmem=20G,slots=2 -smp 6 \
    -kernel  ./../linux-stable/arch/x86/boot/bzImage \
    -append 'root=/dev/vdb3 rw rootfstype=ext4 data=ordered 
rcupdate.rcu_expedited=1 pci=lastbus=0 tsc=reliable no_timer_check reboot=t 
noapictimer console=hvc0' \
    -no-user-config -nodefaults \
    -drive file=/home/chen/project/clear-18080-kvm.img,if=virtio,format=raw 
-nographic -monitor pty \
    -device virtio-serial-pci,id=virtio-serial0,disable-modern \
    -chardev stdio,id=charconsole0 \
    -device virtconsole,chardev=charconsole0,id=console \
    -chardev socket,id=char0,path=/var/tmp/vhost.1 \
    -device 
vhost-user-blk-pci,chardev=char0,num-queues=1,bootindex=2,config-wce=true \
    -object 
memory-backend-file,id=mem1,size=1G,mem-path=/dev/hugepages,share=on \
    -numa node,memdev=mem1

When I set config-wce=true or false, the below value never change
 root@unicorn ~ # cat /sys/block/vda/cache_type
write back
root@unicorn ~ # cat /sys/block/vda/device/features
0010 0010 0110 0110 0000 0000 0000 1000 1000 0000 0000 0000 0000 0000 0000 0000 
>From left to right, bit 11=0, which show there is no WCE feature in 
>vhost-user-blk device.

I also did one simple patch to add config_wce value into s->blkcfg.wce, but the 
result is same.

diff --git a/hw/block/vhost-user-blk.c b/hw/block/vhost-user-blk.c
index 9cb61336a6..0e3f490c82 100644
--- a/hw/block/vhost-user-blk.c
+++ b/hw/block/vhost-user-blk.c
@@ -447,6 +447,9 @@ reconnect:
         s->blkcfg.num_queues = s->num_queues;
     }

+    if (s->blkcfg.wce != s->config_wce) {
+        s->blkcfg.wce = s->config_wce;
+    }
     return;

It seems the config-wce property setting in Qemu is not working, and I am not 
sure if this is a bug or vhost-user-blk only support write back mode ?
Thanks a lot!

Regards,

Yang