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Re: [Xen-devel] baud rate of qemu serial port

To: James Harper <james.harper@xxxxxxxxxxxxxxxx>, xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] baud rate of qemu serial port
From: Trolle Selander <trolle.selander@xxxxxxxxx>
Date: Sat, 16 Oct 2010 22:11:15 -0400
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Hi James,

It is rate limited. There is a timer in qemu-dm which runs at a rate decided by the baud rate, and which takes bytes from the virtual FIFO and sends them on to the "back end" device, so it doesn't matter if it's a pty or a physical serial port. It would be pretty easy to change this to allow for theoretically higher speeds, but the result would probably be pretty disappointing; I did experiment a bit with this when I wrote the 16550A patch, and if I remember right, at the time you pretty much maxed out at about ~200kbit. Now, that was a couple of years ago, and the speeds reachable now might be quite a bit higher, due to CPUs having gotten faster, as well as VMEXIT being far "lighter" on newer generation CPUs.


On Sat, Oct 16, 2010 at 9:24 PM, James Harper <james.harper@xxxxxxxxxxxxxxxx> wrote:
If I specify serial='pty' in my config for a HVM domU, does the baud
rate that the virtual hardware gets set to actually make a different to
the throughput or is it not rate limited in any way?



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