Re: [Xen-devel] update_runstate_area and Linux KPTI

[Juergen Gross <jgross@xxxxxxxx>]

On 02/03/18 17:51, Jan Beulich wrote:
>>>> On 02.03.18 at 17:25, <julien.grall@xxxxxxx> 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?
> 
> 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.


Juergen

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