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Re: [Xen-devel] [PATCH] Console Daemon

To: "Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] Console Daemon
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Wed, 03 Aug 2005 13:41:08 +0100
Cc: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>, Rusty Russell <rusty@xxxxxxxxxxxxxxx>, Ian Pratt <Ian.Pratt@xxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Robert Read <robert@xxxxxxxxxxxxx>
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> It only really needs to be big enough to absorb all the messages before
> Linux's console buffering code comes up, which isn't very much,
> certainly <4KB.
> My view is that we should continue using the same buffer once xenbus
> comes up, hence there should be a simple multiplexing protocol.  

The kernel will end up allocating a buffer per console anyway, usually
at least a page in size. May as well just make that buffer visible via
xenstore and have console daemon directly pull the bytes out of it.

This also avoids the need for framing infomation, so no packet-ising
of data within the console ring buffers, which also makes recovery and
suspend/resume somewhat easier (no need to resync ring indexes -- the
new/restarted daemon can just start reading out of the ring buffer
where the previous daemon left off with no potentially fragile
recover/restart protocol).

Bottom line: I'd like simple, correct and highly available console
ahead of features like multi-headed console driver or xenbus-enabled

 -- Keir

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