Re: [Xen-devel] [PATCH 3 of 5] rombios/ata: Reading this status register has no relevant side effects

[Alan Cox <alan@xxxxxxxxxxxxxxxxxxx>]

On Mon, 30 Jul 2012 20:47:22 +0100
Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:

> So taking two traps when one will do is pointless.  This codepath is on the 
> int
> 0x13 hot path, and removing it has about a 30% reduction in the number of 
> traps
> to Qemu during Win7 boot.

You can't read the status for 400nS after a command issue, so throwing
one away is a typical way to handle that.

All of this is optimising the wrong thing.

The problem is that neither kvm not xen have the most basic prediction
handlers in the kernel side exception code so keep hitting qemu.

For a 99% of the ATA transfers you can predict the next few in and outs
and pre-load them into your trap handler avoiding bouncing into qemu, on
a miss you go back into qemu and load the next prediction block (or tree
even)

That's the kind of optimisation that will really make it fly.

Alan


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