Re: [PATCH v3 2/3] spapr: nvdimm: Implement H_SCM_FLUSH hcall

From: Shivaprasad G Bhat <sbhat@linux.ibm.com>
To: David Gibson <david@gibson.dropbear.id.au>,
	"Aneesh Kumar K.V" <aneesh.kumar@linux.ibm.com>
Cc: 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: Fri, 26 Mar 2021 19:15:32 +0530	[thread overview]
Message-ID: <8c642adb-7c07-41e1-07d0-f23bb6c2f865@linux.ibm.com> (raw)
In-Reply-To: <YFvsmKiXtb+h9HBO@yekko.fritz.box>

On 3/25/21 7:21 AM, David Gibson wrote:
> 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.
...
>>>> 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?

virtio-pmem device has virtqueue with virtio_pmem_flush() as the handler

which gets called for all flush requests from guest. virtio_pmem_flush() is

offloading the flush to thread pool with a worker doing fsync() and the

completion callback notifying the guest with response.

>> 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?

Is it possible to have different defaults for sync-dax based on 
architecture ?

The behaviour on x86 is sync-dax=on for nvdimms. So, it would be correct to

have the default as "on" for x86. For pseries -  "off" for new machines.

Looking at code, I didnt find much ways to achieve this. Can you suggest

what can be done ?
_______________________________________________
Linux-nvdimm mailing list -- linux-nvdimm@lists.01.org
To unsubscribe send an email to linux-nvdimm-leave@lists.01.org