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Subject: Re: [virtio-comment] [PATCH 2/5] virtio-blk spec: writeback cache enable improvements
Il 19/08/2013 21:10, Michael S. Tsirkin ha scritto: > virtio-blk is widely used. I wouldn't write it off just yet. > So I think these comments are valuable. > > 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? No, it isn't. Very much the contrary, actually. > We really should send scsi > commands to the device to figure out and set/clear it. Absolutely not, because this would bypass the host kernel. Flush requests would not work properly. There is _no_ reason why a guest kernel should _ever_ use VIRTIO_BLK_F_SCSI, except if userspace tells it to (with the assumption that userspace knows what it's doing). In fact, deprecating VIRTIO_BLK_F_SCSI would be a very good idea. It is half-assed, as Rusty put it, confusing, and mostly useless. It's also one of the few cases where we rely on buffers to determine field sizes. > I also note that virtio-blk simply has custom sysfs attributes > for WCE control and status. Is this normal? Yes, the sysfs attribute is modeled after the one for SCSI disks. > James, are there tools that send scsi commands to block devices > to control WCE? You can use sg_raw, but it is not safe. Not coincidentially, the sg3_utils suite has a tool to send MODE SENSE (i.e. grab the raw state from the block device), but no tool to send MODE SELECT. Paolo
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