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

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: dunlapg@gmail.com [mailto:dunlapg@gmail.com] 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@lists.xen.org; Stefano Stabellini;
> zhiyuan.lv@intel.com; 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@citrix.com>
> 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.

  Paul

> 
>  -George