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

[Keir Fraser <keir@xxxxxxxxxxxxx>]

Yes, it strikes me that this cannot be done safely without providing a set of ‘proxy device models’ in the hypervisor that know when it is safe to buffer and when the buffers must be flushed, according to native hardware behaviour.

 -- Keir

On 21/7/07 11:59, "Trolle Selander" <trolle.selander@xxxxxxxxx> wrote:

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


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