linux-ext4 - Re: [PATCH v3 0/5] kvm "virtio pmem" device

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Date:   Wed, 09 Jan 2019 21:40:02 -0500
From:   Rik van Riel <riel@...riel.com>
To:     Dave Chinner <david@...morbit.com>,
        Pankaj Gupta <pagupta@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        qemu-devel@...gnu.org, linux-nvdimm@...1.01.org,
        linux-fsdevel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        linux-acpi@...r.kernel.org, linux-ext4@...r.kernel.org,
        linux-xfs@...r.kernel.org, jack@...e.cz, stefanha@...hat.com,
        dan.j.williams@...el.com, nilal@...hat.com, kwolf@...hat.com,
        pbonzini@...hat.com, zwisler@...nel.org, vishal.l.verma@...el.com,
        dave.jiang@...el.com, david@...hat.com, jmoyer@...hat.com,
        xiaoguangrong.eric@...il.com, hch@...radead.org, mst@...hat.com,
        jasowang@...hat.com, lcapitulino@...hat.com, imammedo@...hat.com,
        eblake@...hat.com, willy@...radead.org, tytso@....edu,
        adilger.kernel@...ger.ca, darrick.wong@...cle.com,
        rjw@...ysocki.net
Subject: Re: [PATCH v3 0/5] kvm "virtio pmem" device

On Thu, 2019-01-10 at 12:26 +1100, Dave Chinner wrote:
> On Wed, Jan 09, 2019 at 08:17:31PM +0530, Pankaj Gupta wrote:
> >  This patch series has implementation for "virtio pmem". 
> >  "virtio pmem" is fake persistent memory(nvdimm) in guest 
> >  which allows to bypass the guest page cache. This also
> >  implements a VIRTIO based asynchronous flush mechanism.  
> 
> Hmmmm. Sharing the host page cache direct into the guest VM. Sounds
> like a good idea, but.....
> 
> This means the guest VM can now run timing attacks to observe host
> side page cache residency, and depending on the implementation I'm
> guessing that the guest will be able to control host side page
> cache eviction, too (e.g. via discard or hole punch operations).

Even if the guest can only access its own disk image,
using its own page cache pages, Pankaj's patches could
still be a win because they allow the host kernel to
easily evict page cache pages without doing IO.

Evicting page cache pages that live inside a guest
involves something like swapping, ballooning, or some
sort of callback into the guest to figure out whether
a page is clean.

Evicting page cache pages that live in the host, OTOH...

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