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Subject: Re: [virtio-comment] [PATCH 2/5] virtio-blk spec: writeback cache enable improvements
Il 20/08/2013 16:13, James Bottomley ha scritto: > > For example, this made me realize that even when scsi command > > pass-through is enabled, we either take the clue on WCE support from > > host feature bit. This is wrong, isn't it? We really should send scsi > > commands to the device to figure out and set/clear it. > > Right: this illustrates how the incremental addition can be done: do > pass through on the mode sense SCSI uses to decide this and discovery > can proceed normally. Older virtio says I don't know what you're > talking about and SCSI assumes write through ... the same will work for > all the other features. No, please don't!!! The host is _not_ supposed to have a SCSI target implementation for VIRTIO_BLK_F_SCSI's sake, no matter how simple! And the guest is not supposed to build CDBs if you are using virtio-blk (if you are writing a Windows driver, you have to interpret CDBs, but that's a different story). If you want CDBs, that's perfect---but then your device model should follow the virtio-scsi spec and reuse _all_ the SCSI code in the guest (virtio-scsi.c and sd.c). Do not try to resurrect the utterly broken VIRTIO_BLK_F_SCSI experiment that unluckily made it to the spec. And keep virtio-blk damn simple. If that means not adding a feature, do not add it. >> > I also note that virtio-blk simply has custom sysfs attributes >> > for WCE control and status. Is this normal? >> > James, are there tools that send scsi commands to block devices >> > to control WCE? > Yes, it's actually very simple > > echo "cache type" > /sys/class/scsi_disk/<dev>/cache_type Same for virtio-blk, though the change is always volatile. Paolo
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