Re: [virtio-comment] [PATCH 2/5] virtio-blk spec: writeback cache enable improvements

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/08/2013 18:51, James Bottomley ha scritto:
>> No, it cannot.  First of all, it only works for virtio-blk devices
>> backed by a physical disk (not a logical volume, not a partition, not a
>> file).  That's a feature, not a bug, since we do not want virtio-blk
>> devices to carry a SCSI target implementation (QEMU's is 2.5 kSLOC).
> 
> The unstated assumption above is that SCSI emulators are huge.  This is
> bogus.  The tiny USB key you probably have in your pocket at the moment
> that you bought for a tenner has a minimal SCSI emulator inside its rom.

And you know how many off-by-one errors or "liberal" implementations of
the standards they contain, and how much code is in sd.c (or in USB
quirks) to handle those problems.

A simple virtio-scsi implementation could use a small SCSI emulator.
But it could still be twice the size of virtio-blk, perhaps.

> You don't need to emulate every command.  Just those that provide the
> features you want.
> 
>> Second, any change you make through SCSI requests is not visible to the
>> host.  For example, if the guest kernel used SCSI requests to enable
>> writeback caching, the host would not know about it and will not send
>> flushes (thus a power loss will eat data).
> 
> I'm a bit lost here.  If you mean the native case, so host passes
> commands directly to underlying SCSI, then, of course, you're correct,
> the host driver doesn't snoop.  But the command chain is unbroken from
> guest to physical device, so there's no data loss on power failure.  If
> you mean the emulated case, then the host has to intercept the command.

VIRTIO_BLK_F_SCSI is not meant to do emulation, since you can perfectly
use virtio-scsi for that usecase.

Paolo