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

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Date:   Wed, 4 Dec 2019 14:52:30 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

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