Messages in this thread | | | Subject | Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking | From | Jason Wang <> | Date | Thu, 5 Dec 2019 14:51:15 +0800 |
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On 2019/12/5 上午3:52, 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: >>>> + 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...
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, >
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