Hi Andrew,

On 03/02/2018 05:09 PM, Andrew Cooper wrote:
On 02/03/18 17:05, Juergen Gross wrote:
On 02/03/18 17:51, Jan Beulich wrote:
On 02.03.18 at 17:25, <julien.gr...@arm.com> wrote:
On 02/03/18 16:18, Jan Beulich wrote:
On 02.03.18 at 17:04, <andrew.coop...@citrix.com> wrote:
The proper way to do this is indeed by a nominated (guest) physical
address, at which point Xen can make all/any updates at times of its
choosing, and the guests pagetable/permissions state at an instantaneous
moment don't matter.

If you've got time to do this, then please do.  It will be a definite
improvement.
Just to be avoid unnecessary effort in the wrong direction: I don't
think you can alter the current interface. You'd have to add a new
one, and we could then deprecate (but never abandon) the current
one.
I was only planning to store the guest physical address rather than the
virtual address as we do today. Is that considered as an alteration of
the current interface?
Yes, it is, as an existing PV kernel could deliberately alter the
mappings underlying the linear address it has handed us.
Linux pvops kernel isn't doing this. Mini-OS neither. I guess kernel-xen
would be okay with this, too. And I bet BSD is also fine.

Seriously: any kernel playing such tricks is asking for problems.

We shouldn't support operation modes which make no sense just for the
sake of compatibility, IMO.

I'd love to do this, but we cant.  Older Linux used to have a virtual
buffer spanning a page boundary.  Changing the behaviour under that will
cause older setups to explode.

For confirmation, by older you mean any current Linux? Looking at them the runstate is coming from the percpu region. I can't find in the code anything promising to have the runstate page-aligned. Did I miss anything?

Cheers,

--
Julien Grall

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