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Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

> From: Paul Durrant [mailto:Paul.Durrant@xxxxxxxxxx]
> Sent: Friday, February 05, 2016 7:24 PM
> > -----Original Message-----
> > From: dunlapg@xxxxxxxxx [mailto:dunlapg@xxxxxxxxx] On Behalf Of
> > George Dunlap
> > Sent: 05 February 2016 11:14
> > To: Paul Durrant
> > Cc: Jan Beulich; George Dunlap; Kevin Tian; Wei Liu; Ian Campbell; Andrew
> > Cooper; Zhang Yu; xen-devel@xxxxxxxxxxxxx; Stefano Stabellini;
> > zhiyuan.lv@xxxxxxxxx; Ian Jackson; Keir (Xen.org)
> > Subject: Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter
> > max_wp_ram_ranges.
> >
> > On Fri, Feb 5, 2016 at 9:24 AM, Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> > wrote:
> > > Utilizing the default server is a backwards step. GVT-g would have to use
> > the old HVM_PARAM mechanism to cause it's emulator to become default. I
> > think a more appropriate mechanism would be p2m_mmio_write_dm to
> > become something like 'p2m_ioreq_server_write' and then have a hypercall
> > to allow it to be mapped to a particular ioreq server.
> > > Obviously only one could claim it but, with a p2t, the bit could be re-
> > purposed to simply mean 'go look in the p2t' for more information and then
> > the p2t could be structured to allow emulations to be steered to one of many
> > ioreq servers (for read and/or write emulation).
> >
> > Right; I had in mind that Xen would allow at any given time a max of N
> > ioreq servers to register for mmio_write_dm ranges, first-come
> > first-served; with 'N' being '1' to begin with.  If a second ioreq
> > server requested mmio_write_dm functionality, it would get -EBUSY.
> > This would allow their current setup (one qemu dm which doesn't do
> > mmio_write_dm, one xengt dm which does) to work without needing to
> > worry any more about how many pages might need to be tracked (either
> > for efficiency or correctness).
> >
> > We could then extend this to some larger number (4 seems pretty
> > reasonable to me) either by adding an extra 3 types, or by some other
> > method (such as the one Paul suggests).
> I think it would be best to do away with the 'write dm' name though. I would 
> like to see it
> be possible to steer reads+writes, as well as writes (and maybe just reads?) 
> to a particular
> ioreq server based on type information. So maybe we just call the existing 
> type
> 'p2m_ioreq_server' and then, in the absence of a p2t, hardcode this to go to 
> whichever
> emulator makes the new TBD hypercall.
> I think we need a proper design at this point. Given that it's Chinese New 
> Year maybe I'll
> have a stab in Yu's absence.

Hi, Paul, what about your progress on this?

My feeling is that we do not need a new hypercall to explicitly claim
whether a ioreq server wants to handle write requests. It can be
implicitly marked upon whether a specific page is requested for
write-protection through a specific ioreq channel, and then that
ioreq server will claim the attribute automatically. 

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