Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

Peter Xu
2019-12-04T19:52:42Z

On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
> On 04/12/19 11:38, Jason Wang wrote:
> >>
> >> +    entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> >> +    entry->slot = slot;
> >> +    entry->offset = offset;
> >
> >
> > Haven't gone through the whole series, sorry if it was a silly question
> > but I wonder things like this will suffer from similar issue on
> > virtually tagged archs as mentioned in [1].
>
> There is no new infrastructure to track the dirty pages---it's just a
> different way to pass them to userspace.
>
> > Is this better to allocate the ring from userspace and set to KVM
> > instead? Then we can use copy_to/from_user() friends (a little bit slow
> > on recent CPUs).
>
> Yeah, I don't think that would be better than mmap.

Yeah I agree, because I didn't see how copy_to/from_user() helped to
do icache/dcache flushings...

Some context here: Jason raised this question offlist first on whether
we should also need these flush_dcache_cache() helpers for operations
like kvm dirty ring accesses. I feel like it should, however I've got
two other questions, on:

- if we need to do flush_dcache_page() on kernel modified pages
(assuming the same page has mapped to userspace), then why don't
we need flush_cache_page() too on the page, where
flush_cache_page() is defined not-a-nop on those archs?

- assuming an arch has not-a-nop impl for flush_[d]cache_page(),
would atomic operations like cmpxchg really work for them
(assuming that ISAs like cmpxchg should depend on cache
consistency).

Sorry I think these are for sure a bit out of topic for kvm dirty ring
patchset, but since we're at it, I'm raising the questions up in case
there're answers..

Thanks,

--
Peter Xu