Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
Date:   Sun, 18 Oct 2020 11:52:06 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     "Jason A. Donenfeld" <Jason@zx2c4.com>
Cc:     Jann Horn <jannh@google.com>, Willy Tarreau <w@1wt.eu>,
        Colm MacCarthaigh <colmmacc@amazon.com>,
        "Catangiu, Adrian Costin" <acatan@amazon.com>,
        Andy Lutomirski <luto@kernel.org>,
        "Theodore Y. Ts'o" <tytso@mit.edu>,
        Eric Biggers <ebiggers@kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        kernel list <linux-kernel@vger.kernel.org>,
        "open list:VIRTIO GPU DRIVER" 
        <virtualization@lists.linux-foundation.org>,
        "Graf (AWS), Alexander" <graf@amazon.de>,
        "Woodhouse, David" <dwmw@amazon.co.uk>, bonzini@gnu.org,
        "Singh, Balbir" <sblbir@amazon.com>,
        "Weiss, Radu" <raduweis@amazon.com>, oridgar@gmail.com,
        ghammer@redhat.com, Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Qemu Developers <qemu-devel@nongnu.org>,
        KVM list <kvm@vger.kernel.org>,
        Michal Hocko <mhocko@kernel.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        Pavel Machek <pavel@ucw.cz>,
        Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH] drivers/virt: vmgenid: add vm generation id driver
Message-ID: <20201018114625-mutt-send-email-mst@kernel.org>
References: <CAG48ez0EanBvDyfthe+hAP0OC8iGLNSq2e5wJVz-=ENNGF97_w@mail.gmail.com>
 <20201017033606.GA14014@1wt.eu>
 <CAG48ez0x2S9XuCrANAQbXNi8Jjwm822-fnQSmr-Zr07JgrEs1g@mail.gmail.com>
 <6CC3DB03-27BA-4F5E-8ADA-BE605D83A85C@amazon.com>
 <CAG48ez1ZtvjOs2CEq8-EMosPCd_o7WQ3Mz_+1mDe7OrH2arxFA@mail.gmail.com>
 <20201017053712.GA14105@1wt.eu>
 <CAG48ez1h0ynXfGap_KiHiPVTfcB8NBQJ-2dnj08ZNfuhrW0jWA@mail.gmail.com>
 <20201017064442.GA14117@1wt.eu>
 <CAG48ez3pXLC+eqAXDCniM0a+5yP2XJODDkZqiUTZUOttCE_LbA@mail.gmail.com>
 <CAHmME9qHGSF8w3DoyCP+ud_N0MAJ5_8zsUWx=rxQB1mFnGcu9w@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CAHmME9qHGSF8w3DoyCP+ud_N0MAJ5_8zsUWx=rxQB1mFnGcu9w@mail.gmail.com>
Precedence: bulk

On Sat, Oct 17, 2020 at 03:24:08PM +0200, Jason A. Donenfeld wrote:
> 4c. The guest kernel maintains an array of physical addresses that are
> MADV_WIPEONFORK. The hypervisor knows about this array and its
> location through whatever protocol, and before resuming a
> moved/snapshotted/duplicated VM, it takes the responsibility for
> memzeroing this memory. The huge pro here would be that this
> eliminates all races, and reduces complexity quite a bit, because the
> hypervisor can perfectly synchronize its bringup (and SMP bringup)
> with this, and it can even optimize things like on-disk memory
> snapshots to simply not write out those pages to disk.
> 
> A 4c-like approach seems like it'd be a lot of bang for the buck -- we
> reuse the existing mechanism (MADV_WIPEONFORK), so there's no new
> userspace API to deal with, and it'd be race free, and eliminate a lot
> of kernel complexity.

Clearly this has a chance to break applications, right?
If there's an app that uses this as a non-system-calls way
to find out whether there was a fork, it will break
when wipe triggers without a fork ...
For example, imagine:

MADV_WIPEONFORK
copy secret data to MADV_DONTFORK
fork


used to work, with this change it gets 0s instead of the secret data.


I am also not sure it's wise to expose each guest process
to the hypervisor like this. E.g. each process needs a
guest physical address of its own then. This is a finite resource.


The mmap interface proposed here is somewhat baroque, but it is
certainly simple to implement ...

-- 
MST