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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2012-11-26 at 16:56 +0000, Andrew Cooper wrote:
> On 26/11/12 16:51, Ian Campbell wrote:
> > No relevant side effects according to some spec or according to the
> > current qemu implementation?
> >
> > Does it have any "irrelevant" side effects?
> 
> I am not sure whether the spec gurarentees no side effects, but this is
> according to the qemu implementation.

This sort of thing is a critical part of the commit message, for the
benefit of some future bug hunter if qemu ever changes this. Although to
be honest this possibility makes me rather sceptical of this sort of
change in the first place.

>   The codepath has no side effects
> we could determine.
> 
> ~Andrew
> 
> >
> > On Mon, 2012-11-26 at 16:41 +0000, Andrew Cooper 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.
> >>
> >> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> >>
> >> diff -r c223c044afbf -r a8645d836a0e tools/firmware/rombios/rombios.c
> >> --- a/tools/firmware/rombios/rombios.c
> >> +++ b/tools/firmware/rombios/rombios.c
> >> @@ -2540,7 +2540,6 @@ static int await_ide(when_done,base,time
> >>    Bit32u time=0,last=0;
> >>    Bit16u status;
> >>    Bit8u result;
> >> -  status = inb(base + ATA_CB_STAT); // for the times you're supposed to 
> >> throw one away
> >>    for(;;) {
> >>      status = inb(base+ATA_CB_STAT);
> >>      time++;
> >>
> >> _______________________________________________
> >> Xen-devel mailing list
> >> Xen-devel@xxxxxxxxxxxxx
> >> http://lists.xen.org/xen-devel
> >
> 



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