Re: update_runstate_area and Linux KPTI

[Andrew Cooper <andrew.cooper3@citrix.com>]

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@arm.com> wrote:
>>> On 02/03/18 16:18, Jan Beulich wrote:
>>>>>>> On 02.03.18 at 17:04, <andrew.cooper3@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.

~Andrew

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