Re: [PATCH V7 0/2] Add handling of extended regions (safe ranges) on Arm (Was "xen/memory: Introduce a hypercall to provide unallocated space")

[Ian Jackson <iwj@xxxxxxxxxxxxxx>]

Oleksandr Tyshchenko writes ("[PATCH V7 0/2] Add handling of extended regions 
(safe ranges) on Arm (Was "xen/memory: Introduce a hypercall to provide 
unallocated space")"):
> You can find an initial discussion at [1]-[7].
> 
> The extended region (safe range) is a region of guest physical address space
> which is unused and could be safely used to create grant/foreign mappings 
> instead
> of wasting real RAM pages from the domain memory for establishing these 
> mappings.

Thanks.

This patch has all the required acks, but I was aware of an
outstanding concern from Andrew, as set out in his most
recent mail on the subject:
  Subject: Re: [RFC PATCH 1/3] xen: Introduce "gpaddr_bits" field to 
XEN_SYSCTL_physinfo
  Date: Tue, 7 Sep 2021 18:35:47 +0100
  Message-ID: <973f5344-aa10-3ad6-ff02-ad5f358ad279@xxxxxxxxxx>

I think it would be within the process to just commit the patch now,
but I thought it best to check as best I could that we weren't missing
anything.  The process is supposed to support our continuing
development and also our quality, so I aim to do those things.

I reviewed that mail and had a conversation with Julien about it on
irc.  My understanding is that Julien and Oleksandr's intent is that
Andrew's concerns have been addressed, although we don't have a
confirmation of that from Andrew.

In particular, I wanted to convince myself that if in fact there was
still a problem, we hadn't made a problem for ourselves with the API
here.

The new hypercalls are in unstable interfaces, so if we need to change
them in a future version (eg to make ARM migration work) that's OK.
Julien tells me that he doesn't believe there to be any impact on the
(x86) migration stream right now.

There is a new libxl stable interface.  But I think it is
inoffensive.  In particular, basically any mechanism to do this would
have that API.  And that doesn't seem to touch on the implementation
issues described by Andrew.

Therefore, I think (i) we have tried to address the issues (ii) any
reminaing problems can be dealt with as followups, without trouble.

So I have just pushed these two.

Thanks,
Ian.