Re: [Qemu-devel] [PATCH] exec: Skip mru section if it's a partial page and not resolving subpage

[Paolo Bonzini <pbonzini@redhat.com>]

On 15/11/2017 20:00, BALATON Zoltan wrote:
> On Wed, 15 Nov 2017, Paolo Bonzini wrote:
>> This is another possibility:
>>
>> diff --git a/exec.c b/exec.c
>> index 97a24a875e..3bb9fcf257 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -410,22 +410,16 @@ static MemoryRegionSection
>> *address_space_lookup_region(AddressSpaceDispatch *d,
>> {
>>     MemoryRegionSection *section = atomic_read(&d->mru_section);
>>     subpage_t *subpage;
>> -    bool update;
>>
>> -    if (section && section !=
>> &d->map.sections[PHYS_SECTION_UNASSIGNED] &&
>> -        section_covers_addr(section, addr)) {
>> -        update = false;
>> -    } else {
>> +    if (!section || section ==
>> &d->map.sections[PHYS_SECTION_UNASSIGNED] ||
>> +        !section_covers_addr(section, addr)) {
>>         section = phys_page_find(d, addr);
>> -        update = true;
>> +        atomic_set(&d->mru_section, section);
>>     }
>>     if (resolve_subpage && section->mr->subpage) {
>>         subpage = container_of(section->mr, subpage_t, iomem);
>>         section =
>> &d->map.sections[subpage->sub_section[SUBPAGE_IDX(addr)]];
>>     }
>> -    if (update) {
>> -        atomic_set(&d->mru_section, section);
>> -    }
>>     return section;
>> }
>>
>>
>> It will skip the expensive phys_page_find but not the cheap subpage
>> lookup.
>> Does it work for you?
> 
> This also works but I can't understand why becuase the problematic call
> to this function which got the wrong result from section_covers_addr was
> with resolve_subpage=false and in that case this looks identical to the
> original which did not work.

The problem was that a resolve_subpage=true mru_section was reused for a
resolve_subpage=false mru_section.  This is fixed by always making the
mru_section the same as if resolve_subpage=false (mru_section is set
before looking into the subpage_t).

Thanks,

Paolo

> Unless this function is called multiple
> times and another call with resolve_subpage=true now somehow works
> better with this change or replacing the mru_section earlier before
> subpage lookup makes a difference which is not obvious from this function.
> 
> Anyway, it fixes the problem I see so you can use this instead of my patch.
> 
> Thank you,
> BALATON Zoltan