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Subject: Re: [PATCH v4 3/3] nvdimm: Enable sync-dax device property for nvdimm
To: Shivaprasad G Bhat <sbhat@linux.ibm.com>, david@gibson.dropbear.id.au,
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References: <161966810162.652.13723419108625443430.stgit@17be908f7c1c>
 <161966813983.652.5749368609701495826.stgit@17be908f7c1c>
From: Eric Blake <eblake@redhat.com>
Organization: Red Hat, Inc.
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Date: Mon, 3 May 2021 13:27:22 -0500
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On 4/28/21 10:49 PM, Shivaprasad G Bhat wrote:
> The patch adds the 'sync-dax' property to the nvdimm device.
> 
> When the sync-dax is 'direct' indicates the backend is synchronous DAX
> capable and no explicit flush requests are required. When the mode is
> set to 'writeback' it indicates the backend is not synhronous DAX

synchronous

> capable and explicit flushes to Hypervisor are required.
> 
> On PPC where the flush requests from guest can be honoured by the qemu,

s/the qemu/qemu/

> the 'writeback' mode is supported and set as the default. The device
> tree property "hcall-flush-required" is added to the nvdimm node which
> makes the guest to issue H_SCM_FLUSH hcalls to request for flushes

s/to issue/issue/
s/request for/request/

> explicitly. This would be the default behaviour without sync-dax
> property set for the nvdimm device. For old pSeries machine, the
> default is 'unsafe'.
> 
> For non-PPC platforms, the mode is set to 'unsafe' as the default.
> 
> Signed-off-by: Shivaprasad G Bhat <sbhat@linux.ibm.com>
> ---

> +++ b/qapi/common.json
> @@ -197,3 +197,23 @@
>  { 'enum': 'GrabToggleKeys',
>    'data': [ 'ctrl-ctrl', 'alt-alt', 'shift-shift','meta-meta', 'scrolllock',
>              'ctrl-scrolllock' ] }
> +
> +##
> +# @NvdimmSyncModes:
> +#
> +# Indicates the mode of flush to be used to ensure persistence in case
> +# of power failures.
> +#
> +# @unsafe: This is to indicate, the data on the backend device not be
> +#          consistent in power failure scenarios.

s/This is to indicate, the/This indicates that/
s/device not/device might not/

> +# @direct: This is to indicate the backend device supports synchronous DAX
> +#          and no explicit flush requests from the guest is required.

This indicates the backend device supports synchronous DAX, and no
explicit flush requests from the guest are required.

> +# @writeback: To be used when the backend device doesn't support synchronous
> +#             DAX. The hypervisor issues flushes to the disk when requested
> +#             by the guest.
> +# Since: 6.0

6.1

> +#
> +##
> +{ 'enum': 'NvdimmSyncModes',
> +  'data': [ 'unsafe', 'writeback',
> +            { 'name': 'direct', 'if': 'defined(CONFIG_LIBPMEM)' } ] }
> 
> 

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org
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