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

[David Vrabel <dvrabel@xxxxxxxxxx>]

On 30/07/12 22:57, Alan Cox wrote:
> 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.

This is only relevant when talking to real hardware, the qemu model has
no such requirement.

Also, I think you mean 400 ns not 400 nanosiemens.

> 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.

I'd be interested in seeing how you think this will work without
knowledge of the emulated device in the hypervisor.  How does the
predictor know whether accesses have side effects?

A better solution would be to avoid most I/O accesses by the BIOS by
using PV drivers instead.

David

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