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

[Jason Wang]

On 2019/12/5 äå3:52, Peter Xu wrote:
> On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
> > On 04/12/19 11:38, Jason Wang wrote:
> > > > +ÂÂÂ entry = &ring->dirty_gfns[ring->dirty_index & (ring->size - 1)];
> > > > +ÂÂÂ entry->slot = slot;
> > > > +ÂÂÂ 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...


It looks to me one advantage is that exact the same VA is used by both 
userspace and kernel so there will be no alias.

Thanks


> 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,