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From: Pankaj Gupta <pagupta@redhat.com>
To: Jan Kara <jack@suse.cz>
Cc: Dan Williams <dan.j.williams@intel.com>,
	KVM list <kvm@vger.kernel.org>,
	David Hildenbrand <david@redhat.com>,
	linux-nvdimm <linux-nvdimm@ml01.01.org>,
	Jason Wang <jasowang@redhat.com>,
	Dave Chinner <david@fromorbit.com>,
	Qemu Developers <qemu-devel@nongnu.org>,
	virtualization@lists.linux-foundation.org,
	adilger kernel <adilger.kernel@dilger.ca>,
	Ross Zwisler <zwisler@kernel.org>,
	dave jiang <dave.jiang@intel.com>,
	darrick wong <darrick.wong@oracle.com>,
	vishal l verma <vishal.l.verma@intel.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Christoph Hellwig <hch@infradead.org>,
	Linux ACPI <linux-acpi@vger.kernel.org>,
	jmoyer <jmoyer@redhat.com>,
	linux-ext4 <linux-ext4@vger.kernel.org>,
	Rik van Riel <riel@surriel.com>,
	Stefan Hajnoczi <stefanha@redhat.com>,
	Igor Mammedov <imammedo@redhat.com>,
	lcapitulino@redhat.com, Kevin Wolf <kwolf@redhat.com>,
	Nitesh Narayan Lal <nilal@redhat.com>,
	Theodore Ts'o <tytso@mit.edu>,
	xiaoguangrong eric <xiaoguangrong.eric@gmail.com>,
	"Rafael J. Wysocki" <rjw@rjwysocki.net>,
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	Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device
Date: Mon, 14 Jan 2019 05:16:55 -0500 (EST)	[thread overview]
Message-ID: <2011860338.64100587.1547461015822.JavaMail.zimbra@redhat.com> (raw)
Message-ID: <20190114101655.8PpkPsn2HAk6dW_PO5P1lrsf6qDpfLdfiDiBub0glZ4@z> (raw)
In-Reply-To: <20190114095520.GC13316@quack2.suse.cz>


> > > > > Right. Thinking about this I would be more concerned about the fact
> > > > > that
> > > > > guest can effectively pin amount of host's page cache upto size of
> > > > > the
> > > > > device/file passed to guest as PMEM, can't it Pankaj? Or is there
> > > > > some
> > > > > QEMU
> > > > > magic that avoids this?
> > > >
> > > > Yes, guest will pin these host page cache pages using 'get_user_pages'
> > > > by
> > > > elevating the page reference count. But these pages can be reclaimed by
> > > > host
> > > > at any time when there is memory pressure.
> > > 
> > > Wait, how can the guest pin the host pages? I would expect this to
> > > happen only when using vfio and device assignment. Otherwise, no the
> > > host can't reclaim a pinned page, that's the whole point of a pin to
> > > prevent the mm from reclaiming ownership.
> > 
> > yes. You are right I just used the pin word but it does not actually pin
> > pages
> > permanently. I had gone through the discussion on existing problems with
> > get_user_pages and DMA e.g [1] to understand Jan's POV. It does mention GUP
> > pin pages so I also used the word 'pin'. But guest does not permanently pin
> > these pages and these pages can be reclaimed by host.
> 
> OK, then I was just confused how virtio-pmem is going to work. Thanks for
> explanation! So can I imagine this as guest mmaping the host file and
> providing the mapped range as "NVDIMM pages" to the kernel inside the
> guest? Or is it more complex?

yes, that's correct. Host's Qemu process virtual address range is used as guest physical 
address and a direct mapping(EPT/NPT) is established. At guest side, this physical memory
range is plugged into guest system memory map and DAX mapping is setup using nvdimm calls.

Thanks,
Pankaj

  parent reply	other threads:[~2019-01-14 10:17 UTC|newest]

Thread overview: 62+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-01-09 14:47 [PATCH v3 0/5] kvm "virtio pmem" device Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 1/5] libnvdimm: nd_region flush callback support Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 2/5] virtio-pmem: Add virtio pmem driver Pankaj Gupta
2019-01-14 15:54   ` Michael S. Tsirkin
2019-01-14 15:54     ` Michael S. Tsirkin
     [not found]     ` <20190114105314-mutt-send-email-mst-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2019-01-15  6:33       ` Pankaj Gupta
2019-01-15  6:33         ` Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 3/5] libnvdimm: add nd_region buffered dax_dev flag Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 4/5] ext4: disable map_sync for virtio pmem Pankaj Gupta
2019-01-09 14:47 ` [PATCH v3 5/5] xfs: " Pankaj Gupta
2019-01-09 14:47   ` Pankaj Gupta
2019-01-09 16:26   ` Darrick J. Wong
2019-01-09 18:08     ` Pankaj Gupta
     [not found] ` <20190109144736.17452-1-pagupta-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-10  1:26   ` [PATCH v3 0/5] kvm "virtio pmem" device Dave Chinner
2019-01-10  1:26     ` Dave Chinner
2019-01-10  2:40     ` Rik van Riel
2019-01-10  2:40       ` Rik van Riel
2019-01-10 10:17     ` Jan Kara
2019-01-10 10:17       ` Jan Kara
     [not found]       ` <20190110101757.GC15790-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org>
2019-01-13  1:38         ` Pankaj Gupta
2019-01-13  1:38           ` Pankaj Gupta
     [not found]           ` <1354249849.63357171.1547343519970.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-13  1:43             ` Dan Williams
2019-01-13  1:43               ` Dan Williams
     [not found]               ` <CAPcyv4hwcgTUpgNCefCGu4DvgkYBp5b=f+hJ+FC=s5APYKoycg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-01-13  2:17                 ` [Qemu-devel] " Pankaj Gupta
2019-01-13  2:17                   ` Pankaj Gupta
     [not found]                   ` <540171952.63371441.1547345866585.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-14  9:55                     ` Jan Kara
2019-01-14  9:55                       ` Jan Kara
     [not found]                       ` <20190114095520.GC13316-4I4JzKEfoa/jFM9bn6wA6Q@public.gmane.org>
2019-01-14 10:16                         ` Pankaj Gupta [this message]
2019-01-14 10:16                           ` Pankaj Gupta
2019-01-11  7:45     ` Pankaj Gupta
2019-01-11  7:45       ` Pankaj Gupta
     [not found]       ` <1326478078.61913951.1547192704870.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-13 23:29         ` Dave Chinner
2019-01-13 23:29           ` Dave Chinner
2019-01-13 23:38           ` Matthew Wilcox
2019-01-13 23:38             ` Matthew Wilcox
     [not found]             ` <20190113233820.GX6310-PfSpb0PWhxZc2C7mugBRk2EX/6BAtgUQ@public.gmane.org>
2019-01-14  2:50               ` Dave Chinner
2019-01-14  2:50                 ` Dave Chinner
2019-01-14  7:15               ` Pankaj Gupta
2019-01-14  7:15                 ` Pankaj Gupta
     [not found]                 ` <942065073.64011540.1547450140670.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-14 21:25                   ` Dave Chinner
2019-01-14 21:25                     ` Dave Chinner
2019-01-14 21:35                     ` Dan Williams
2019-01-14 21:35                       ` Dan Williams
     [not found]                       ` <CAPcyv4jtPcLV-s0sKNHwwk0ug7GLBV6699dpm1h3r2xSo879dg-JsoAwUIsXosN+BqQ9rBEUg@public.gmane.org>
2019-01-14 22:21                         ` Dave Chinner
2019-01-14 22:21                           ` Dave Chinner
2019-01-15  2:19                           ` Michael S. Tsirkin
2019-01-15  2:19                             ` Michael S. Tsirkin
     [not found]                             ` <20190114205031-mutt-send-email-mst-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>
2019-01-15  5:37                               ` [Qemu-devel] " Pankaj Gupta
2019-01-15  5:37                                 ` Pankaj Gupta
2019-01-15  5:35                           ` Pankaj Gupta
2019-01-15  5:35                             ` Pankaj Gupta
     [not found]                             ` <1684638419.64320214.1547530506805.JavaMail.zimbra-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
2019-01-15 20:42                               ` Dave Chinner
2019-01-15 20:42                                 ` Dave Chinner
2019-02-04 22:56 ` security implications of caching with virtio pmem (was Re: [PATCH v3 0/5] kvm "virtio pmem" device) Michael S. Tsirkin
2019-02-05  7:29   ` [Qemu-devel] " Pankaj Gupta
2019-02-06 14:00   ` David Hildenbrand
2019-02-06 18:01     ` Michael S. Tsirkin
2019-02-11  7:29   ` [Qemu-devel] " Pankaj Gupta
2019-02-11 22:29     ` Dave Chinner
2019-02-11 22:58       ` David Hildenbrand
2019-02-11 23:07         ` Michael S. Tsirkin
  -- strict thread matches above, loose matches on Subject: below --
2019-01-09 13:50 [PATCH v3 0/5] kvm "virtio pmem" device Pankaj Gupta
2019-01-09 14:46 ` [Qemu-devel] " Pankaj Gupta

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