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Date:   Tue, 10 Dec 2019 08:25:43 -0500
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Paolo Bonzini <pbonzini@redhat.com>
Cc:     Jason Wang <jasowang@redhat.com>, Peter Xu <peterx@redhat.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>
Subject: Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking
Message-ID: <20191210081958-mutt-send-email-mst@kernel.org>
References: <20191129213505.18472-1-peterx@redhat.com>
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 <a3e83e6b-4bfa-3a6b-4b43-5dd451e03254@redhat.com>
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On Wed, Dec 04, 2019 at 12:04:53PM +0100, Paolo Bonzini wrote:
> On 04/12/19 11:38, Jason Wang wrote:
> >>
> >> +=A0=A0=A0 entry =3D &ring->dirty_gfns[ring->dirty_index & (ring->size=
 - 1)];
> >> +=A0=A0=A0 entry->slot =3D slot;
> >> +=A0=A0=A0 entry->offset =3D offset;
> >=20
> >=20
> > 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].
>=20
> There is no new infrastructure to track the dirty pages---it's just a
> different way to pass them to userspace.

Did you guys consider using one of the virtio ring formats?
Maybe reusing vhost code?

If you did and it's not a good fit, this is something good to mention
in the commit log.

I also wonder about performance numbers - any data here?


> > 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).
>=20
> Yeah, I don't think that would be better than mmap.
>=20
> Paolo
>=20
>=20
> > [1] https://lkml.org/lkml/2019/4/9/5