[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] Require that xenstored writes to a domain complete in a single chunk

  • To: David Edmondson <dme@xxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • From: Keir Fraser <keir@xxxxxxxxxxxxx>
  • Date: Mon, 26 Feb 2007 17:20:34 +0000
  • Delivery-date: Mon, 26 Feb 2007 09:19:55 -0800
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>
  • Thread-index: AcdZymxiqv/X9MW9Edu1UwAX8io7RQ==
  • Thread-topic: [Xen-devel] [PATCH] Require that xenstored writes to a domain complete in a single chunk

On 26/2/07 16:24, "David Edmondson" <dme@xxxxxxx> wrote:

> If xenstored is part-way through writing a header+payload into the
> buffer shared with a guest domain when the guest domain decides to
> suspend, the buffer is corrupted, as xenstored doesn't know that it
> has a partial write to complete when the domain revives.  The domain
> is expecting proper completion of the partial header+payload and is
> disappointed.
> The attached patch modifies xenstored such that it checks for
> sufficient space for header+payload before making any changes to the
> shared buffer.
> It is against 3.0.4-1, but the code in unstable looks the same.

This seems dubious. There's no reason we might not have payloads bigger than
the ring size (which is only 1kB).

The right fix would be in the guest, which should already be stopping any
transactions or commands across save/restore. Does this problem occur when
xenstored sends an asynchronous watch-fired message? Probably the
packet-reading thread should be interrupted and put to sleep before

For older guest compatibility perhaps we can take a variant of your patch
that only waits for enough space is the entire message fits in the ring in
one go. This would be 'best-effort' at compatibility while not precluding
use of larger messages in general.

 -- Keir

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.