Re: [Xen-devel] [xen-unstable test] 104131: regressions - FAIL

["Tian, Kevin" <kevin.tian@xxxxxxxxx>]

> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Wednesday, January 18, 2017 5:38 PM
> 
> >>> On 18.01.17 at 05:57, <kevin.tian@xxxxxxxxx> wrote:
> > Attached was my earlier comment:
> >
> > --
> >> >>> On 20.12.16 at 06:37, <kevin.tian@xxxxxxxxx> wrote:
> >> >>  From: Xuquan (Quan Xu) [mailto:xuquan8@xxxxxxxxxx]
> >> >> Sent: Friday, December 16, 2016 5:40 PM
> >> >> -        if (pt_vector != -1)
> >> >> -            vmx_set_eoi_exit_bitmap(v, pt_vector);
> >> >> +        if ( pt_vector != -1 ) {
> >> >> +            if ( intack.vector > pt_vector )
> >> >> +                vmx_set_eoi_exit_bitmap(v, intack.vector);
> >> >> +            else
> >> >> +                vmx_set_eoi_exit_bitmap(v, pt_vector);
> >> >> +        }
> >> >
> >> > Above can be simplified as one line change:
> >> >  if ( pt_vector != -1 )
> >> >          vmx_set_eoi_exit_bitmap(v, intack.vector);
> >>
> >> Hmm, I don't understand. Did you mean to use max() here? Or
> >> else how is this an equivalent of the originally proposed code?
> >>
> >
> > Original code is not 100% correct. The purpose is to set EOI exit
> > bitmap for any vector which may block injection of pt_vector -
> > give chance to recognize pt_vector in future intack and then do pt
> > intr post. The simplified code achieves this effect same as original
> > code if intack.vector >= vector. I cannot come up a case why
> > intack.vector might be smaller than vector. If this case happens,
> > we still need enable exit bitmap for intack.vector instead of
> > pt_vector for said purpose while original code did it wrong.
> >
> > Thanks
> > Kevin
> > --
> >
> > Using intack.vector is always expected here regardless of the
> > comparison result between intack.vector and pt_vector. The
> > reason why I was OK adding an ASSERT was simply to test
> > whether intack.vecor<pt_vector does happen which is
> > orthogonal to the fix itself.
> 
> Well, a vector lower than pt_vector can't block delivery. Or wait:

There are two points here:

a) We need enable EOI exit bitmap when pt_vector is blocked.

b) As you said, ideally a vector lower than pt_vecotr cannot block

The patch fixed a) and then added an ASSERT to verify b). Strictly
speaking they are separate issues.

> Don't we need to consider vector classes here, i.e.
> 
>             ASSERT((intack.vector >> 4) >= (pt_vector >> 4));
> 
> ?
> 

However it still doesn't explain why original ASSERT is triggered.
vlapic_find_highest_vector actually finds the highest vector, instead
of highest class...

static int vlapic_find_highest_vector(const void *bitmap)
{
    const uint32_t *word = bitmap;
    unsigned int word_offset = NR_VECTORS / 32;

    /* Work backwards through the bitmap (first 32-bit word in every four). */
    while ( (word_offset != 0) && (word[(--word_offset)*4] == 0) )
        continue;

    return (fls(word[word_offset*4]) - 1) + (word_offset * 32);
}

Thanks
Kevin

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