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SubjectRe: [PATCH v5 13/13] KVM: Optimize overlapping memslots check
On Wed, Oct 27, 2021, Maciej S. Szmigiero wrote:
> On 26.10.2021 20:59, Sean Christopherson wrote:
> > > + /* kvm_for_each_in_gfn_no_more() guarantees that cslot->base_gfn < nend */
> > > + if (cend > nslot->base_gfn)
> >
> > Hmm, IMO the need for this check means that kvm_for_each_memslot_in_gfn_range()
> > is flawed. The user of kvm_for_each...() should not be responsible for skipping
> > memslots that do not actually overlap the requested range. I.e. this function
> > should be no more than:
> >
> > static bool kvm_check_memslot_overlap(struct kvm_memslots *slots,
> > struct kvm_memory_slot *slot)
> > {
> > gfn_t start = slot->base_gfn;
> > gfn_t end = start + slot->npages;
> >
> > kvm_for_each_memslot_in_gfn_range(&iter, slots, start, end) {
> > if (iter.slot->id != slot->id)
> > return true;
> > }
> >
> > return false;
> > }
> >
> >
> > and I suspect kvm_zap_gfn_range() could be further simplified as well.
> >
> > Looking back at the introduction of the helper, its comment's highlighting of
> > "possibily" now makes sense.
> >
> > /* Iterate over each memslot *possibly* intersecting [start, end) range */
> > #define kvm_for_each_memslot_in_gfn_range(node, slots, start, end) \
> >
> > That's an unnecessarily bad API. It's a very solvable problem for the iterator
> > helpers to advance until there's actually overlap, not doing so violates the
> > principle of least surprise, and unless I'm missing something, there's no use
> > case for an "approximate" iteration.
> In principle this can be done, however this will complicate the gfn
> iterator logic - especially the kvm_memslot_iter_start() part, which
> will already get messier from open-coding kvm_memslots_gfn_upper_bound()
> there.

Hmm, no, this is trivial to handle, though admittedly a bit unpleasant.

* Note, kvm_memslot_iter_start() finds the first memslot that _may_ overlap
* the range, it does not verify that there is actual overlap. The check in
* the loop body filters out the case where the highest memslot with a base_gfn
* below start doesn't actually overlap.
#define kvm_for_each_memslot_in_gfn_range(iter, node, slots, start, end) \
for (kvm_memslot_iter_start(iter, node, slots, start, end); \
kvm_memslot_iter_is_valid(iter); \
kvm_memslot_iter_next(node)) \
if (iter->slot->base_gfn + iter->slot->npages < start) { \
} else

> At the same kvm_zap_gfn_range() will still need to do the memslot range
> <-> request range merging by itself as it does not want to process the
> whole returned memslot, but rather just the part that's actually
> overlapping its requested range.

That's purely coincidental though. IMO, kvm_zap_gfn_range() would be well within
its rights to sanity the memslot, e.g.

if (WARN_ON(memslot->base_gfn + memslot->npages < gfn_start))

> In the worst case, the current code can return one memslot too much, so
> I don't think it's worth bringing additional complexity just to detect
> and skip it

I strongly disagree. This is very much a case of one chunk of code that knows
the internal details of what it's doing taking on all the pain and complexity
so that users of the helper

> it's not that uncommon to design an API that needs extra checking from its
> caller to cover some corner cases.

That doesn't mean it's desirable.

> For example, see pthread_cond_wait() or kernel waitqueues with their
> spurious wakeups or atomic_compare_exchange_weak() from C11.
> And these are higher level APIs than a very limited internal KVM one
> with just two callers.

Two _existing_ callers. Odds are very, very high that future usage of
kvm_for_each_memslot_in_gfn_range() will overlook the detail about the helper
not actually doing what it says it does. That could be addressed to some extent
by renaming it kvm_for_each_memslot_in_gfn_range_approx() or whatever, but as
above this isn't difficult to handle, just gross.

> In case of kvm_zap_gfn_range() the necessary checking is already
> there and has to be kept due to the above range merging.
> Also, a code that is simpler is easier to understand, maintain and
> so less prone to subtle bugs.

Heh, and IMO that's an argument for putting all the complexity into a single
location. :-)

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