Re: Runstate hypercall and Linux KPTI issues

[Bertrand Marquis <Bertrand.Marquis@arm.com>]

Hi,

Sorry for the delay.

> On 11 Sep 2020, at 01:33, Stefano Stabellini <sstabellini@kernel.org> wrote:
>
> On Thu, 10 Sep 2020, Jan Beulich wrote:
>>>>> - should we backport the support for this hypercall in older kernel releases ?
>>>>
>>>> It's a bug fix to KPTI, and as such ought to be at least eligible for
>>>> considering doing so?
>>>
>>> That will mean also backport in Linux. What should be the scope ?
>>
>> All longterm and stable trees which are affected, as I think is usual.
>
> From a Linux perspective, we should fix this as soon as possible: we
> should backport a patch to make the usage of the runstate hypercall
> conditional on KPTI being disabled on ARM.
>
> Then, when available in Xen, we should backport the usage of the new
> hypercall with a check to detect if it is available -- do not assume it
> is available, users might not update Xen, but might update Linux.
>
> We have to do this in two stages for a couple of reasons. It is best not
> to wait for Xen-side changes to fix Linux any longer. And also, a Linux
> fix is best implemented independently anyway: Linux should benefit from
> the Xen improvements when available but not rely on their presence to
> work.

I fully agree with that and this should solve the short term issue.

In the long term the new hypercall seems to be the only solution but
it is not a small change and I will not have time to work on this in the
near future.

Cheers
Bertrand




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