linux-kernel - Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory tracking

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Date:   Mon, 20 Jan 2020 02:47:46 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Christophe de Dinechin <dinechin@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Yan Zhao <yan.y.zhao@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Kevin Kevin <kevin.tian@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Lei Cao <lei.cao@...atus.com>
Subject: Re: [PATCH v3 12/21] KVM: X86: Implement ring-based dirty memory
 tracking

On Mon, Jan 20, 2020 at 03:29:15PM +0800, Peter Xu wrote:
> On Sun, Jan 19, 2020 at 05:12:35AM -0500, Michael S. Tsirkin wrote:
> > On Sun, Jan 19, 2020 at 10:09:53AM +0100, Paolo Bonzini wrote:
> > > On 09/01/20 20:15, Peter Xu wrote:
> > > > Regarding dropping the indices: I feel like it can be done, though we
> > > > probably need two extra bits for each GFN entry, for example:
> > > > 
> > > >   - Bit 0 of the GFN address to show whether this is a valid publish
> > > >     of dirty gfn
> > > > 
> > > >   - Bit 1 of the GFN address to show whether this is collected by the
> > > >     user
> > > 
> > > We can use bit 62 and 63 of the GFN.
> > 
> > If we are short on bits we can just use 1 bit. E.g. set if
> > userspace has collected the GFN.
> 
> I'm still unsure whether we can use only one bit for this.  Say,
> otherwise how does the userspace knows the entry is valid?  For
> example, the entry with all zeros ({.slot = 0, gfn = 0}) could be
> recognized as a valid dirty page on slot 0 gfn 0, even if it's
> actually an unused entry.

So I guess the reverse: valid entry has bit set, userspace sets it to
0 when it collects it?


> > 
> > > I think this can be done in a secure way.  Later in the thread you say:
> > > 
> > > > We simply check fetch_index (sorry I
> > > > meant this when I said reset_index, anyway it's the only index that we
> > > > expose to userspace) to make sure:
> > > > 
> > > >   reset_index <= fetch_index <= dirty_index
> > > 
> > > So this means that KVM_RESET_DIRTY_RINGS should only test the "collected
> > > by user" flag on dirty ring entries between reset_index and dirty_index.
> > > 
> > > Also I would make it
> > > 
> > >    00b (invalid GFN) ->
> > >      01b (valid gfn published by kernel, which is dirty) ->
> > >        1*b (gfn dirty page collected by userspace) ->
> > >          00b (gfn reset by kernel, so goes back to invalid gfn)
> > > That is 10b and 11b are equivalent.  The kernel doesn't read that bit if
> > > userspace has collected the page.
> 
> Yes "1*b" is good too (IMHO as long as we can define three states for
> an entry).  However do you want me to change to that?  Note that I
> still think we need to read the rest of the field (in this case,
> "slot" and "gfn") besides the two bits to do re-protect.  Should we
> trust that unconditionally if writable?
> 
> Thanks,
> 
> -- 
> Peter Xu