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[Xen-devel] Re: [Xen-staging] [xen-unstable] [QEMU-DM] Upgrade emulated UART to 16550A.
- To: "Alex Williamson" <alex.williamson@xxxxxx>
- From: "Trolle Selander" <trolle.selander@xxxxxxxxx>
- Date: Wed, 5 Dec 2007 21:30:17 +0100
- Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
- Delivery-date: Wed, 05 Dec 2007 12:30:53 -0800
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As you describe this, it does occur to me as you describe this that serial_xmit() never gives up on trying to retransmit characters out of the FIFO, which isn't very good for backends like pty's or pipes. I'll fix it asap.
On Dec 5, 2007 9:15 PM, Alex Williamson < alex.williamson@xxxxxx> wrote:
On Wed, 2007-12-05 at 14:22 +0000, Xen staging patchbot-unstable wrote:
> # HG changeset patch
> # User Keir Fraser
> # Date 1196864460 0
> # Node ID f0ac46de680cc6fe8c91699fdda153b125ae515c
> # Parent bf21e00155b7dd76653c5340099ecedac7a7de08
> [QEMU-DM] Upgrade emulated UART to 16550A.
>
> This patch adds 16550 emulation to qemu-dm. I still consider it a work
> in progress, but from my testing, it consistently performs better than
> the old code already, sometimes considerably so (on my laptop, dumping
> out data over serial from a HVM DomU -> pty in Dom0 was up to 5.3
> times faster than with the old 16450 code). I can consistenly reach
> full 115200 baud speeds over physical serial lines when the virtual
> device is backed by a phys port, which I never could with the old code.
>
> There are still some things to be done. I want to add proper error
> handling, so that overflow/framing & parity errors on the physical
> port get detected and reported by the virtual port. Also, now that
> FIFOs are in place, I believe performance could be improved
> significantly by putting some code into the hypervisor so that reads &
> writes from the FIFOs don't have to exit into qemu-dm at all. I'm also
> reading up on the specs for newer uarts with deeper FIFOs.
I notice with this patch that my HVM domain, which specifies
serial=pty in the config file, no longer boots without me attaching to
the console via xm. Previously it would boot regardless of whether I
was attached to the console. Is this change in behavior intentional?
Note that I'm testing on ia64 where the HVM firmware spews lots of debug
info out onto the serial port. Maybe this new UART model isn't dropping
transmit chars when there's no client connected as it did before?
Thanks,
Alex
--
Alex Williamson HP Open Source & Linux Org.
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