Re: [Xen-devel] Buffered IO for IO?

["Trolle Selander" <trolle.selander@xxxxxxxxx>]

Safety would depend on how the emulated device works. For serial ports
in particular, it's definitely not safe, since depending on the model
of UART emulated, and the settings of the UART control registers, every
outb may result in a serial interrupt and UART register changes that
will have to be processed before any further io can be done. 
It's possible that there might be some performance to be gained
by "upgrading" the emulated UART to a 16550A or better, and doing
buffered IO for the FIFO. Earlier this year I was experimenting with a
patch that made the qemu-dm serial emulation into a 16550A with FIFO,
but though the patch did fix some compatability issues with software
that assumed a 16550A UART in the HVM guest I'm working with, serial
performance actually got noticeably _worse_, so I never bothered
submitting it. Implementing the FIFO with buffered IO would possibly
make it work better, but I don't see how it could be done without
moving at least part of the serial device model into the hypervisor,
which just strikes me as more trouble than it's worth.

/Trolle

On 7/21/07, Keir Fraser <keir@xxxxxxxxxxxxx> wrote:

On 20/7/07 22:33, "Zulauf, John" <john.zulauf@xxxxxxxxx> wrote:

> Has anyone experimented with adding Buffered IO support for "out"
> instructions?  Currently, the buffered io pages is only used for mmio
> writes (and then only to vga space).  It seems quite straight-forward to
> add.

Is it safe to buffer, and hence arbitrarily delay, any I/O port write?

-- Keir


_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxx
http://lists.xensource.com/xen-devel