RE: [PATCH v2 3/3] [SCSI] Make LBP quirk skip lbpme checks tests

[KY Srinivasan <kys@microsoft.com>]

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> -----Original Message-----
> From: James Bottomley [mailto:jbottomley@parallels.com]
> Sent: Friday, July 25, 2014 10:10 AM
> To: KY Srinivasan
> Cc: linux-kernel@vger.kernel.org; hch@infradead.org; sitsofe@gmail.com;
> devel@linuxdriverproject.org; apw@canonical.com;
> martin.petersen@oracle.com; linux-scsi@vger.kernel.org;
> ohering@suse.com; gregkh@linuxfoundation.org; jasowang@redhat.com
> Subject: Re: [PATCH v2 3/3] [SCSI] Make LBP quirk skip lbpme checks tests
> 
> On Fri, 2014-07-25 at 16:47 +0000, KY Srinivasan wrote:
> >
> > > -----Original Message-----
> > > From: Martin K. Petersen [mailto:martin.petersen@oracle.com]
> > > Sent: Thursday, July 24, 2014 8:54 AM
> > > To: Sitsofe Wheeler
> > > Cc: Martin K. Petersen; Christoph Hellwig; KY Srinivasan;
> > > gregkh@linuxfoundation.org; linux-kernel@vger.kernel.org;
> > > devel@linuxdriverproject.org; ohering@suse.com; apw@canonical.com;
> > > jasowang@redhat.com; jbottomley@parallels.com; linux-
> > > scsi@vger.kernel.org
> > > Subject: Re: [PATCH v2 3/3] [SCSI] Make LBP quirk skip lbpme checks
> > > tests
> > >
> > > >>>>> "Sitsofe" == Sitsofe Wheeler <sitsofe@gmail.com> writes:
> > >
> > > Sitsofe> So we can see it is really a SATA device that announces
> > > Sitsofe> discard correctly and supports discard through WRITE_SAME(16).
> > >
> > > No, that's the SATA device that announces support for DSM TRIM, and
> > > as a result the Linux SATL reports support for WRITE SAME(16) w. the
> > > UNMAP bit set and LBPME.
> > >
> > > Sitsofe> It is the act of passing it through Hyper-V that turned it
> > > Sitsofe> into a SCSI device that supports UNMAP (but not
> > > Sitsofe> WRITE_SAME(16)), doesn't announce its SCSI conformance
> > > Sitsofe> number and doesn't correctly announce which features it
> > > Sitsofe> supports. Surely in this case it's reasonable to quirk our way
> around the problem?
> > >
> > > No. That's an issue in Hyper-V that'll you'll have to take up with
> > > Microsoft. I don't know what their passthrough limitations are for SCSI-
> ATA translation.
> > > Maybe K. Y. has some insight into this?
> >
> > For the pass through case, the host validates the request and passes
> > the request to the device.
> > However, not all scsi commands are passed through even though the
> > device it is being passed through may support the command. WRITE_SAME
> > is one such command. Consequently, in the EVPD page, we will set state
> > indicating that WRITE_SAME is not supported (even if the device
> > supports it).
> 
> I think you haven't appreciated the problem: He's passing a SATA SSD via the
> SCSI hyper-v interface.  That means that the windows host is doing SCSI<-
> >ATA translation.  The problem is that the Windows translation layer (SATL)
> looks to be incomplete and it's not correctly translating the IDENTIFY bit that
> corresponds to TRIM to the correct VPD pages so consequently, Linux  won't
> send UNMAP commands to the device (to be translated back to TRIM).
> 
> We already know this is a bug in the Windows SATL which needs fixing (if you
> could report it and get a fix, that would be great) and that we're not going to
> be able to work around this automatically in Linux because the proposed
> patch would have us unconditionally try UNMAP for all Hyper-V devices.  The
> current proposed fix is to enable UNMAP manually via sysfs in the guest boot
> scripts, but obviously that means that Hyper-V guests with direct pass
> through of SSDs need operator intervention to turn on TRIM.

James,

Thanks for the clarification. I am talking to the folks in MSFT that develop the native scsi
stack on Windows. Hyper-V's back-end driver is not involved in SATL. I will keep you guys
posted.

Regards,

K. Y
> 
> James

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