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

> -----Original Message-----
> From: dunlapg@xxxxxxxxx [mailto:dunlapg@xxxxxxxxx] On Behalf Of
> George Dunlap
> Sent: 22 February 2016 15:56
> To: Paul Durrant
> Cc: George Dunlap; Jan Beulich; Kevin Tian; Zhang Yu; Wei Liu; Ian Campbell;
> Andrew Cooper; xen-devel@xxxxxxxxxxxxx; Stefano Stabellini; Zhiyuan Lv; Ian
> Jackson; Keir (Xen.org)
> Subject: Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter
> max_wp_ram_ranges.
> On Wed, Feb 17, 2016 at 11:12 AM, Paul Durrant <Paul.Durrant@xxxxxxxxxx>
> wrote:
> >> -----Original Message-----
> >> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> >> Sent: 17 February 2016 11:02
> >> To: Jan Beulich; Paul Durrant; Kevin Tian; Zhang Yu
> >> Cc: Andrew Cooper; Ian Campbell; Ian Jackson; Stefano Stabellini; Wei Liu;
> >> Zhiyuan Lv; xen-devel@xxxxxxxxxxxxx; George Dunlap; Keir (Xen.org)
> >> Subject: Re: [Xen-devel] [PATCH v3 3/3] tools: introduce parameter
> >> max_wp_ram_ranges.
> >>
> >> On 17/02/16 10:22, Jan Beulich wrote:
> >> >>>> On 17.02.16 at 10:58, <kevin.tian@xxxxxxxxx> wrote:
> >> >> Thanks for the help. Let's see whether we can have some solution
> ready
> >> for
> >> >> 4.7. :-)
> >> >
> >> > Since we now seem to all agree that a different approach is going to
> >> > be taken, I think we indeed should revert f5a32c5b8e ("x86/HVM:
> >> > differentiate IO/mem resources tracked by ioreq server"). Please
> >> > voice objections to the plan pretty soon.
> >>
> >> FWIW, after this discussion, I don't have an objection to the basic
> >> interface in this series as it is, since it addresses my request that it
> >> be memory-based, and it could be switched to using p2m types
> >> behind-the-scenes -- with the exception of the knob to control how many
> >> ranges are allowed (since it exposes the internal implementation).
> >>
> >> I wouldn't object to the current implementation going in as it was in v9
> >> (apparently), and then changing the type stuff around behind the scenes
> >> later as an optimization.  I also don't think it would be terribly
> >> difficult to change the implementation as it is to just use write_dm for
> >> a single IOREQ server.  We can rename it ioreq_server and expand it
> >> later.  Sorry if this wasn't clear from my comments before.
> >>
> >
> > The problem is that, since you objected to wp_mem being treated the
> same as mmio, we had to introduce a new range type to the map/unmap
> hypercalls and that will become redundant if we steer by type. If we could
> have just increased the size of the rangeset for mmio and used that *for
> now* then weeks of work could have been saved going down this dead end.
> So first of all, let me say that I handled things respect to this
> thread rather poorly in a lot of ways, and you're right to be
> frustrated.  All I can say is, I'm sorry and I'll try to do better.
> I'm not sure I understand what you're saying with this line: "We had
> to introduce a new range type to the map/unmap hypercalls that will
> become redundant if we steer by type".

What I means is that if we are going to do away with the concept of 'wp mem' 
and replace it with 'special type for steering to ioreq server' then we only 
need range types of pci, portio and mmio as we had before.
> The interface you have here simply says, "Please allow the guest to
> read from these gpfns, but give me (an ioreq server) a notification if
> the guest attempts to write to them."  One way to implement this is
> the way you have done -- by taking the existing (basically unused)
> write_dm p2m type, and layering the existing MMIO rangesets on top of
> it.

That type was introduced for this purpose; it was not there before.

>  Another way you could have (and AFAICS still can) implemented
> this interface is by allowing ioreq servers asking for write
> intercepts on gpfns to claim individual p2m types set aside for that
> purpose -- starting with the existing write_dm, which would only allow
> one such server to register, and later perhaps extending to allow
> more.
> In what way would the second implementation make the new type for the
> map/unmap hypercalls redundant?

Because that type is going to go away, now that it has no use.


>  -George
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