On Fri, May 20, 2022 at 12:59:55PM +0200, Jan Beulich wrote:
> On 20.05.2022 12:22, Roger Pau Monné wrote:
> > On Wed, May 18, 2022 at 12:06:29PM +0200, Jan Beulich wrote:
> >> On 06.05.2022 15:25, Roger Pau Monné wrote:
> >>> On Mon, Apr 25, 2022 at 10:41:23AM +0200, Jan Beulich wrote:
> >>>> --- /dev/null
> >>>> +++ b/xen/arch/x86/include/asm/pt-contig-markers.h
> >>>> @@ -0,0 +1,105 @@
> >>>> +#ifndef __ASM_X86_PT_CONTIG_MARKERS_H
> >>>> +#define __ASM_X86_PT_CONTIG_MARKERS_H
> >>>> +
> >>>> +/*
> >>>> + * Short of having function templates in C, the function defined below
> >>>> is
> >>>> + * intended to be used by multiple parties interested in recording the
> >>>> + * degree of contiguity in mappings by a single page table.
> >>>> + *
> >>>> + * Scheme: Every entry records the order of contiguous successive
> >>>> entries,
> >>>> + * up to the maximum order covered by that entry (which is the number of
> >>>> + * clear low bits in its index, with entry 0 being the exception using
> >>>> + * the base-2 logarithm of the number of entries in a single page
> >>>> table).
> >>>> + * While a few entries need touching upon update, knowing whether the
> >>>> + * table is fully contiguous (and can hence be replaced by a higher
> >>>> level
> >>>> + * leaf entry) is then possible by simply looking at entry 0's marker.
> >>>> + *
> >>>> + * Prereqs:
> >>>> + * - CONTIG_MASK needs to be #define-d, to a value having at least 4
> >>>> + * contiguous bits (ignored by hardware), before including this file,
> >>>> + * - page tables to be passed here need to be initialized with correct
> >>>> + * markers.
> >>>
> >>> Not sure it's very relevant, but might we worth adding that:
> >>>
> >>> - Null entries must have the PTE zeroed except for the CONTIG_MASK
> >>> region in order to be considered as inactive.
> >>
> >> NP, I've added an item along these lines.
> >>
> >>>> +static bool pt_update_contig_markers(uint64_t *pt, unsigned int idx,
> >>>> + unsigned int level, enum PTE_kind
> >>>> kind)
> >>>> +{
> >>>> + unsigned int b, i = idx;
> >>>> + unsigned int shift = (level - 1) * CONTIG_LEVEL_SHIFT + PAGE_SHIFT;
> >>>> +
> >>>> + ASSERT(idx < CONTIG_NR);
> >>>> + ASSERT(!(pt[idx] & CONTIG_MASK));
> >>>> +
> >>>> + /* Step 1: Reduce markers in lower numbered entries. */
> >>>> + while ( i )
> >>>> + {
> >>>> + b = find_first_set_bit(i);
> >>>> + i &= ~(1U << b);
> >>>> + if ( GET_MARKER(pt[i]) > b )
> >>>> + SET_MARKER(pt[i], b);
> >>>
> >>> Can't you exit early when you find an entry that already has the
> >>> to-be-set contiguous marker <= b, as lower numbered entries will then
> >>> also be <= b'?
> >>>
> >>> Ie:
> >>>
> >>> if ( GET_MARKER(pt[i]) <= b )
> >>> break;
> >>> else
> >>> SET_MARKER(pt[i], b);
> >>
> >> Almost - I think it would need to be
> >>
> >> if ( GET_MARKER(pt[i]) < b )
> >> break;
> >> if ( GET_MARKER(pt[i]) > b )
> >> SET_MARKER(pt[i], b);
> >
> > I guess I'm slightly confused, but if marker at i is <= b, then all
> > following markers will also be <=, and hence could be skipped?
>
> Your use of "following" is ambiguous here, because the iteration
> moves downwards as far as PTEs inspected are concerned (and it's
> b which grows from one iteration to the next). But yes, I think I
> agree now that ...
Right, 'following' here would be the next item processed by the loop.
> > Not sure why we need to keep iterating if GET_MARKER(pt[i]) == b.
>
> ... this isn't needed. At which point ...
>
> > FWIW, you could even do:
> >
> > if ( GET_MARKER(pt[i]) <= b )
> > break;
> > SET_MARKER(pt[i], b);
> >
> > Which would keep the conditionals to 1 like it currently is.
> >
> >>
> >> or, accepting redundant updates,
> >>
> >> if ( GET_MARKER(pt[i]) < b )
> >> break;
> >> SET_MARKER(pt[i], b);
> >>
> >> . Neither the redundant updates nor the extra (easily mis-predicted)
> >> conditional looked very appealing to me, but I guess I could change
> >> this if you are convinced that's better than continuing a loop with
> >> at most 9 (typically less) iterations.
> >
> > Well, I think I at least partly understood the logic. Not sure
> > whether it's worth adding the conditional or just assuming that
> > continuing the loop is going to be cheaper. Might be worth adding a
> > comment that we choose to explicitly not add an extra conditional to
> > check for early exit, because we assume that to be more expensive than
> > just continuing.
>
> ... this resolves without further action.
OK, since we agree, and that was the only comment I had, you can add:
Reviewed-by: Roger Pau Monné <roger....@citrix.com>
Thanks, Roger.
