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From: David Gibson <david@gibson.dropbear.id.au>
To: "Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: Shivaprasad G Bhat <sbhat@linux.ibm.com>,
	sbhat@linux.vnet.ibm.com, groug@kaod.org, qemu-ppc@nongnu.org,
	ehabkost@redhat.com, marcel.apfelbaum@gmail.com, mst@redhat.com,
	imammedo@redhat.com, xiaoguangrong.eric@gmail.com,
	qemu-devel@nongnu.org, linux-nvdimm@lists.01.org,
	kvm-ppc@vger.kernel.org, shivaprasadbhat@gmail.com,
	bharata@linux.vnet.ibm.com
Subject: Re: [PATCH v3 2/3] spapr: nvdimm: Implement H_SCM_FLUSH hcall
Date: Thu, 25 Mar 2021 12:51:20 +1100	[thread overview]
Message-ID: <YFvsmKiXtb+h9HBO@yekko.fritz.box> (raw)
In-Reply-To: <19b5aa0b-df85-256d-d4c4-eacd0ea8312e@linux.ibm.com>


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On Wed, Mar 24, 2021 at 09:34:06AM +0530, Aneesh Kumar K.V wrote:
> On 3/24/21 8:37 AM, David Gibson wrote:
> > On Tue, Mar 23, 2021 at 09:47:38AM -0400, Shivaprasad G Bhat wrote:
> > > The patch adds support for the SCM flush hcall for the nvdimm devices.
> > > To be available for exploitation by guest through the next patch.
> > > 
> > > The hcall expects the semantics such that the flush to return
> > > with H_BUSY when the operation is expected to take longer time along
> > > with a continue_token. The hcall to be called again providing the
> > > continue_token to get the status. So, all fresh requsts are put into
> > > a 'pending' list and flush worker is submitted to the thread pool.
> > > The thread pool completion callbacks move the requests to 'completed'
> > > list, which are cleaned up after reporting to guest in subsequent
> > > hcalls to get the status.
> > > 
> > > The semantics makes it necessary to preserve the continue_tokens
> > > and their return status even across migrations. So, the pre_save
> > > handler for the device waits for the flush worker to complete and
> > > collects all the hcall states from 'completed' list. The necessary
> > > nvdimm flush specific vmstate structures are added to the spapr
> > > machine vmstate.
> > > 
> > > Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
> > 
> > An overal question: surely the same issue must arise on x86 with
> > file-backed NVDIMMs.  How do they handle this case?
> 
> On x86 we have different ways nvdimm can be discovered. ACPI NFIT, e820 map
> and virtio_pmem. Among these virio_pmem always operated with synchronous dax
> disabled and both ACPI and e820 doesn't have the ability to differentiate
> support for synchronous dax.

Ok.  And for the virtio-pmem case, how are the extra flushes actually
done on x86?

> With that I would expect users to use virtio_pmem when using using file
> backed NVDIMMS

So... should we prevent advertising an NVDIMM through ACPI or e820 if
it doesn't have sync-dax enabled?

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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  reply	other threads:[~2021-03-25  1:52 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-03-23 13:47 [PATCH v3 0/3] spapr: nvdimm: Enable sync-dax property for nvdimm Shivaprasad G Bhat
2021-03-23 13:47 ` [PATCH v3 1/3] spapr: nvdimm: Forward declare and move the definitions Shivaprasad G Bhat
2021-03-24  2:30   ` David Gibson
2021-03-23 13:47 ` [PATCH v3 2/3] spapr: nvdimm: Implement H_SCM_FLUSH hcall Shivaprasad G Bhat
2021-03-24  3:07   ` David Gibson
2021-03-24  4:04     ` Aneesh Kumar K.V
2021-03-25  1:51       ` David Gibson [this message]
2021-03-26 13:45         ` Shivaprasad G Bhat
2021-03-29  9:23     ` Shivaprasad G Bhat
2021-03-30 23:57       ` David Gibson
2021-03-23 13:47 ` [PATCH v3 3/3] spapr: nvdimm: Enable sync-dax device property for nvdimm Shivaprasad G Bhat
2021-03-24  3:09   ` David Gibson
2021-03-24  4:09     ` Aneesh Kumar K.V

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