Re: [PATCH] sg_io: allow UNMAP and WRITE SAME without CAP_SYS_RAWIO

From: Paolo Bonzini
Date: Tue Sep 11 2012 - 15:24:31 EST

Il 11/09/2012 21:13, Tejun Heo ha scritto:
> Hello, Paolo.
> On Tue, Sep 11, 2012 at 08:54:03PM +0200, Paolo Bonzini wrote:
>>> On Tue, Sep 11, 2012 at 07:56:53PM +0200, Paolo Bonzini wrote:
>>>> Understood; unfortunately, there is another major user of it
>>>> (virtualization). If you are passing "raw" LUNs down to a virtual
>>>> machine, there's no possibility at all to use a properly encapsulated
>>> Is there still command filtering issue when you're passing "raw" LUNs
>>> down?
>> Yes, the passing down is just a userland program that gets SCSI
>> commands from the guest, sends them via SG_IO, and passes back the
>> result. If the userland program is unprivileged (it usually is), then
>> you go through the filter.
> Could being able to bypass the filters for this "you own this LUN" be
> a solution? Or is it that we still need command filtering for
> whatever reason?

Yes, it could be. Enabling/disabling the filters from a privileged
program and passing the unfiltered fd via SCM_RIGHTS would be enough.

>> This is the userland for virtio-scsi (the kernel part of virtio-scsi is just
>> a driver running in the guest). It can run in two mode: it can do its own
>> SCSI emulation, or it can just relay CDBs and their results.
>> It can (and does) use higher-level services if SCSI emulation is done in
>> userland. In that case, trim/discard can become a BLKDISCARD or a fallocate
>> for example. However, in this case userland doesn't do any emulation and in
>> fact doesn't even need to know that this CDB is a discard.
> Couldn't it intercept some of them - e.g. RWs and discards?
> What's the benifit / use case of doing pure bypass?

Basically, using the same storage technology for bare metal and
virtualized systems. IMHO losing sense data is a no-no, but the above
solution could be feasible too.

> Would the benefits be strong enough to justify whole bpf cdb filtering?

If we can get a simpler solution that is okay with kernel maintainers,
I'm all for it.

>>> Hmmm? This was about discard, no?
>> One example of block layer interfaces that I want to add is BLKPING, so
>> that you can see if the NAS is reachable. Then SCSI emulation can map
>> the "test unit ready" command to BLKPING. There's a handful of such
>> ioctls that would be useful, such as BLKDISCARD itself.
> Can't you make use of the existing disk events mechanism for that?
> Block layer already knows how to watch readiness of a device and tell
> the userland about it via uevent.

How? But anyway i don't want to divert the discussion from the actual

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