Re: [Qemu-devel] [RFC v2 1/2] virtio: add pmem driver

From: Pankaj Gupta <pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
To: Jeff Moyer <jmoyer-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Dan Williams
	<dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
Cc: Kevin Wolf <kwolf-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Jan Kara <jack-AlSwsSmVLrQ@public.gmane.org>,
	Xiao Guangrong
	<xiaoguangrong.eric-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>,
	KVM list <kvm-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>,
	David Hildenbrand <david-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	linux-nvdimm
	<linux-nvdimm-y27Ovi1pjclAfugRpC6u6w@public.gmane.org>,
	"Michael S. Tsirkin"
	<mst-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Ross Zwisler
	<ross.zwisler-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>,
	Qemu Developers
	<qemu-devel-qX2TKyscuCcdnm+yROfE0A@public.gmane.org>,
	lcapitulino-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org,
	Christoph Hellwig <hch-wEGCiKHe2LqWVfeAwA7xHQ@public.gmane.org>,
	Linux MM <linux-mm-Bw31MaZKKs3YtjvyW6yDsg@public.gmane.org>,
	Igor Mammedov <imammedo-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Stefan Hajnoczi
	<stefanha-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	niteshnarayanlal-PkbjNfxxIARBDgjK7y7TUQ@public.gmane.org,
	Marcel Apfelbaum <marcel-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Paolo Bonzini <pbonzini-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Rik van Riel <riel-ebMLmSuQjDVBDgjK7y7TUQ@public.gmane.org>,
	Nitesh Narayan Lal
	<nilal-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>,
	Linux Kernel Mailing List
	<linux-kernel-u79uwXL29TY76Z2rM5mHXA@public.gmane.org>
Subject: Re: [Qemu-devel] [RFC v2 1/2] virtio: add pmem driver
Date: Thu, 26 Apr 2018 13:15:38 -0400 (EDT)	[thread overview]
Message-ID: <1499190564.23017177.1524762938762.JavaMail.zimbra@redhat.com> (raw)
In-Reply-To: <x49o9i6885e.fsf-RRHT56Q3PSP4kTEheFKJxxDDeQx5vsVwAInAS/Ez/D0@public.gmane.org>

> 
> Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org> writes:
> 
> > [ adding Jeff directly since he has also been looking at
> > infrastructure to track when MAP_SYNC should be disabled ]
> >
> > On Wed, Apr 25, 2018 at 7:21 AM, Dan Williams <dan.j.williams-ral2JQCrhuEAvxtiuMwx3w@public.gmane.org>
> > wrote:
> >> On Wed, Apr 25, 2018 at 4:24 AM, Pankaj Gupta <pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org> wrote:
> >>> This patch adds virtio-pmem driver for KVM
> >>> guest.
> >>
> >> Minor nit, please expand your changelog line wrapping to 72 columns.
> >>
> >>>
> >>> Guest reads the persistent memory range
> >>> information from Qemu over VIRTIO and registers
> >>> it on nvdimm_bus. It also creates a nd_region
> >>> object with the persistent memory range
> >>> information so that existing 'nvdimm/pmem'
> >>> driver can reserve this into system memory map.
> >>> This way 'virtio-pmem' driver uses existing
> >>> functionality of pmem driver to register persistent
> >>> memory compatible for DAX capable filesystems.
> >>
> >> We need some additional enabling to disable MAP_SYNC for this
> 
> enable to disable... I like it!  ;-)
> 
> >> configuration. In other words, if fsync() is required then we must
> >> disable the MAP_SYNC optimization. I think this should be a struct
> >> dax_device property looked up at mmap time in each MAP_SYNC capable
> >> ->mmap() file operation implementation.

I understand you mean we want to disable 'MAP_SYNC' optimization as
we are relying on additional fsync. You mean if we add a property/flag
in dax_device struct and its set, disable 'MAP_SYNC' accordingly during
mmap time for corresponding filesystems?  

> 
> Ideally, qemu (seabios?) would advertise a platform capabilities
> sub-table that doesn't fill in the flush bits.

Could you please elaborate on this, how its related to disabling
MAP_SYNC? We are not doing entire nvdimm device emulation. 

> 
> -Jeff
> 
> 