Re: [Xen-devel] [PATCH 2 of 5] rombios/ata: Do not wait for BSY to be set

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/11/12 17:49, Alan Cox wrote:
> O> SEABios has explicitly removed the wait for the BSY bit to be set.  We
>> want to remove it because Qemu will not set the BSY bit, resulting in
> So why not fix the qemu emulation bug ?

I do not have a copy of the ATA specification, and
http://code.coreboot.org/p/seabios/source/commit/580e33293244fee4556e56ecc67b8bd877f3c496/
certainly implies that waiting for BSY is not required

>
>> 42k needless polled IO traps while we wait for the timeout (which is, if
>> I remember correctly, based on a loop counter rather than any notion of
>> actual time)
> Not good, although half caused by the fact that years on qemu still
> hasn't implemented predictors to avoid trapping further than the kernel
> layer 8)
>
>> We can certainly argue about the outb(0x80,0x00).  When I was comparing
>> ROMBios with SEABios, this outb was the closest I could easily get to a
>> udelay(5) without implement udelay() in ROMBios.  Xen will execute an
>> outb(0x80, 0x00) on the real hardware if a guest executes it; 
> Don't do that then. 0x80 is not safe on a modern PC system (you'll note
> the Linux kernel changed behaviour here). It may be a debug port but we've
> seen some hardware do worse things if you touch it. It's also completely
> out on a limb with EFI based systems which don't necessarily have the
> legacy stuff present.
>
> Alan

If port 0x80 is problematic, then this is a Xen problem, but a legacy
guest should have no adverse effects.  Perhaps we should "emulate" port
0x80 by deliberately re-scheduling another VCPU ?

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com


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