Re: [Xen-devel] update_runstate_area and Linux KPTI

[Juergen Gross <jgross@xxxxxxxx>]

On 02/03/18 17:25, Julien Grall wrote:
> 
> 
> On 02/03/18 16:18, Jan Beulich wrote:
>>>>> On 02.03.18 at 17:04, <andrew.cooper3@xxxxxxxxxx> 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?

I don't think so. It should be perfectly fine to assume the mapping of
the registered virtual address isn't changed by the guest.

> In other words, the current version (e.g store virtual address) is just
> broken and going to be worst with KPTI kernel. I can't see how this
> could ever work properly on OS with different set of page-tables.

map_vcpu_info() seems to be a nice example how this should be done.
This should make update_runstate_area() simpler and faster.


Juergen

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