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Date: Mon, 14 Jan 2019 02:15:40 -0500 (EST)
From: Pankaj Gupta <pagupta@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH v3 0/5] kvm "virtio pmem" device
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To: Matthew Wilcox <willy@infradead.org>
Cc: Dave Chinner <david@fromorbit.com>, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, qemu-devel@nongnu.org, linux-nvdimm@ml01.01.org, linux-fsdevel@vger.kernel.org, virtualization@lists.linux-foundation.org, linux-acpi@vger.kernel.org, linux-ext4@vger.kernel.org, linux-xfs@vger.kernel.org, jack@suse.cz, stefanha@redhat.com, dan j williams <dan.j.williams@intel.com>, riel@surriel.com, nilal@redhat.com, kwolf@redhat.com, pbonzini@redhat.com, zwisler@kernel.org, vishal l verma <vishal.l.verma@intel.com>, dave jiang <dave.jiang@intel.com>, david@redhat.com, jmoyer@redhat.com, xiaoguangrong eric <xiaoguangrong.eric@gmail.com>, hch@infradead.org, mst@redhat.com, jasowang@redhat.com, lcapitulino@redhat.com, imammedo@redhat.com, eblake@redhat.com, tytso@mit.edu, adilger kernel <adilger.kernel@dilger.ca>, darrick wong <darrick.wong@oracle.com>, rjw@rjwysocki.net


> > Until you have images (and hence host page cache) shared between
> > multiple guests. People will want to do this, because it means they
> > only need a single set of pages in host memory for executable
> > binaries rather than a set of pages per guest. Then you have
> > multiple guests being able to detect residency of the same set of
> > pages. If the guests can then, in any way, control eviction of the
> > pages from the host cache, then we have a guest-to-guest information
> > leak channel.
> 
> I don't think we should ever be considering something that would allow a
> guest to evict page's from the host's pagecache [1].  The guest should
> be able to kick its own references to the host's pagecache out of its
> own pagecache, but not be able to influence whether the host or another
> guest has a read-only mapping cached.
> 
> [1] Unless the guest is allowed to modify the host's file; obviously
> truncation, holepunching, etc are going to evict pages from the host's
> page cache.

This is so correct. Guest does not not evict host page cache pages directly. 
In case of virtio-pmem & DAX, guest clears guest page cache exceptional entries.
Its solely decision of host to take action on the host page cache pages.

In case of virtio-pmem, guest does not modify host file directly i.e don't
perform hole punch & truncation operation directly on host file.  

Thanks,
Pankaj