Re: KVM: guest: only batch user pte updates

Jeremy Fitzhardinge
2009-02-10T23:14:50Z

Marcelo Tosatti wrote:
> On Tue, Feb 10, 2009 at 02:17:49PM -0800, Jeremy Fitzhardinge wrote:
>
>> Marcelo Tosatti wrote:
>>
>>> KVM's paravirt mmu pte batching has issues with, at least, kernel
>>> updates from DEBUG_PAGEALLOC.
>>>
>>> This has been experienced with slab allocation from irq context from
>>> within lazy mmu sections:
>>>
>>> https://bugzilla.redhat.com/show_bug.cgi?id=480822
>>>
>>> DEBUG_PAGEALLOC will map/unmap the kernel pagetables to catch bad
>>> accesses, with code such as:
>>>
>>> __change_page_attr():
>>>
>>> /*
>>> * Do we really change anything ?
>>> */
>>> if (pte_val(old_pte) != pte_val(new_pte)) {
>>> set_pte_atomic(kpte, new_pte);
>>> cpa->flags |= CPA_FLUSHTLB;
>>> }
>>>
>>> A present->nonpresent update can be queued, but not yet committed to
>>> memory. So the set_pte_atomic will be skipped but the update flushed
>>> afterwards. set_pte_ATOMIC.
>>>
>>>
>> Are you saying that there's a queued update which means that old_pte is
>> a stale value which happens to equal new_pte, so new_pte is never set?
>> OK, sounds like a generic problem, of the same sort we've had with
>> kmap_atomic being used in interrupt routines in lazy mode.
>>
>
> Yes. It seems however that only set_pte_at/pte_update/_defer are
> used under significatly long lazy mmu sections (long as in number of
> updates). Is it worthwhile to bother (and risk) batching kernel pte updates ?
>

Well, that depends on how expensive each update is. For something like
kunmap atomic, I think combining the clear+tlb flush probably is worthwhile.

> Until someone forgets about arch_flush_lazy_mmu_mode again...
>

It has been surprisingly unproblematic, though this CPA issue came to
light. But given that there's only a few "correct" ways to update the
kernel mappings now (kmap/vmap/vmalloc, kmap_atomic and cpa, I think),
it should be easy to cover all the bases. (Hm, better check vmap.)

J