Re: [Xen-devel] [OSSTEST PATCH 55/62] make-flight: shadow test: Disable kpti in guests

[Ian Jackson <ian.jackson@xxxxxxxxxx>]

Andrew Cooper writes ("Re: [OSSTEST PATCH 55/62] make-flight: shadow test: 
Disable kpti in guests"):
> On 10/04/2019 15:24, Ian Jackson wrote:
> > Since Spectre/Meltdown, shadow has been a lot slower, especially with
> > KPTI in the guest.  Empirically, too slow (with the kernel from Debian
> > stretch).
> 
> The speed of shadow pagetables hasn't changed - I don't think we even
> touched the shadow code at all for XSA-254.
> 
> The problem is the change in guest behaviour as a consequence of needing
> KPTI for a Meltdown mitigation.
> 
> The guest now flushes its pagetables on every
> syscall/interrupt/exception rather than once on a process=>process
> context switch, which is why running a guest using KPTI in shadow mode
> is boarderline unusable.
> 
> The actual change to use nopti looks fine.

Thanks for the explanation.  Unfortunately for the reasons I've
explained I won't change the commit message because that would involve
rewriting the commit, but it is useful to have this correction in the
archives.

Regards,
Ian.

_______________________________________________
Xen-devel mailing list
Xen-devel@xxxxxxxxxxxxxxxxxxxx
https://lists.xenproject.org/mailman/listinfo/xen-devel