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Re: Runstate hypercall and Linux KPTI issues

To: Stefano Stabellini <sstabellini@xxxxxxxxxx>

From: Bertrand Marquis <Bertrand.Marquis@xxxxxxx>

Date: Thu, 24 Sep 2020 17:28:18 +0000

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Cc: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <stefanos@xxxxxxxxxx>, George Dunlap <George.Dunlap@xxxxxxxxxx>, "jgross@xxxxxxxx" <jgross@xxxxxxxx>, nd <nd@xxxxxxx>

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Thread-topic: Runstate hypercall and Linux KPTI issues

> On 24 Sep 2020, at 18:25, Bertrand Marquis <Bertrand.Marquis@xxxxxxx> wrote: > > Hi, > > Sorry for the delay. > >> On 11 Sep 2020, at 01:33, Stefano Stabellini <sstabellini@xxxxxxxxxx> 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 > Sorry for the notice, it seems our mail server is adding it randomly… Bertrand > > > > IMPORTANT NOTICE: The contents of this email and any attachments are > confidential and may also be privileged. If you are not the intended > recipient, please notify the sender immediately and do not disclose the > contents to any other person, use it for any purpose, or store or copy the > information in any medium. Thank you. >

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