Re: [PATCH] scsi: lpfc: Add shutdown method for kexec

From: Brian King <brking@linux.vnet.ibm.com>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Anton Blanchard <anton@samba.org>
Cc: james.smart@broadcom.com, dick.kennedy@broadcom.com,
	jejb@linux.vnet.ibm.com, martin.petersen@oracle.com,
	jk@ozlabs.org, linux-scsi@vger.kernel.org
Subject: Re: [PATCH] scsi: lpfc: Add shutdown method for kexec
Date: Tue, 14 Feb 2017 08:56:15 -0600	[thread overview]
Message-ID: <39054c0e-c0f3-a62d-2c71-469798727bfd@linux.vnet.ibm.com> (raw)
In-Reply-To: <1487037886.23576.16.camel@kernel.crashing.org>

On 02/13/2017 08:04 PM, Benjamin Herrenschmidt wrote:
> On Mon, 2017-02-13 at 15:57 -0600, Brian King wrote:
>> If we do transition to use remove rather than shutdown, I think we
>> want
>> some way for a device driver to know whether we are doing kexec or
>> not.
>> A RAID adapter with a write cache is going to want to flush its write
>> cache on a PCI hotplug remove, but for a kexec, its going to want to
>> skip
>> that so the kexec is faster. Today, since kexec looks like a reboot,
>> rather than a shutdown, we can skip the flush on a reboot, since its
>> technically not needed there either.
> 
> What happens if a non-flushed adapter gets a PERST ?

It depends on the adapter, so it really needs to be a policy in the device
driver. For any adapter that has a non volatile write cache, data must preserved,
otherwise the write cache is not truly non volatile. For adapters with a volatile
write cache, then its subject to the adapter hardware. For ipr adapters
that have a volatile cache, they do guarantee the data is preserved across
a PERST.

> I wouldn't trust that 'don't have to flush' magic ...

I really don't think we want to force RAID adapters that have gigabytes of
*non volatile* write cache to flush their cache when we are merely performing
a kexec. This can take several minutes in the worst case scenarios.

-Brian

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center