Re: [PATCH 5/5] ioeventfd: Introduce KVM_IOEVENTFD_FLAG_SOCKET

Anthony Liguori
2011-07-20T21:10:24Z

On 07/20/2011 04:44 AM, Pekka Enberg wrote:
> On Tue, 2011-07-19 at 21:49 -0500, Anthony Liguori wrote:
>> If you're not going to emulate a UART properly, why not just use a
>> paravirtual serial device that you can keep simple?
>
> We want to emulate it correctly to make it robust. The problems you
> pointed out were caused by me being under heavy dose of ignorance when I
> wrote the emulation code. ;-)

A UART has a fixed frequency associated with it and the divider latch
will select the actual frequency based on a division of the fixed frequency.

There is no way to send a byte over the serial line faster than the
fixed frequency. The UART is so simple that it cannot DMA and it does
not have very much memory on it for buffering so this limitation is
visible to the guest. This means the driver has to be very involved in
the flower control of the device.

There's no way around this. You simply can't do 10gb/s over a serial
line and still have something that looks and acts like a UART.

BTW, it used to be we were pretty loose with how fast we'd move data
through the serial port in QEMU and Linux would actually throw warnings
if it saw more data moving through the serial port than is theoretically
possible on bare metal. Those warnings were removed at some point but
I'm sure if you dug up and kernel and ran it, you would run into them
with your current emulation.

Regards,

Anthony Liguori

>
> Pekka
>