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

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Message-ID: <20191205120800.GA9673@xz-x1>
Date:   Thu, 5 Dec 2019 07:08:00 -0500
From:   Peter Xu <peterx@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...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 Thu, Dec 05, 2019 at 02:51:15PM +0800, Jason Wang wrote:
> 
> 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.

Hmm.. but what if the page is mapped more than once in user?  Thanks,

-- 
Peter Xu