Re: [Xen-devel] update_runstate_area and Linux KPTI

[Juergen Gross <jgross@xxxxxxxx>]

On 02/03/18 18:09, 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.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.
> 
> 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.

Adding a special per-domain mapping for that purpose would work.


Juergen


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