Re: [PATCH] x86/xen: Tolerate nested XEN_LAZY_MMU entering/leaving

[Jürgen Groß]

On 12.05.26 18:05, Marek Marczykowski-Górecki wrote:
> On Fri, May 08, 2026 at 04:39:33PM +0200, Juergen Gross wrote:
> > With the support of nested lazy mmu sections it can happen that
> > arch_enter_lazy_mmu_mode() is being called twice without a call of
> > arch_leave_lazy_mmu_mode() in between, as the lazy_mmu_*() helpers
> > are not disabling preemption when checking for nested lazy mmu
> > sections.
> >
> > This is a problem when running as a Xen PV guest, as
> > xen_enter_lazy_mmu() and xen_leave_lazy_mmu() don't tolerate this
> > case.
> >
> > Fix that in xen_enter_lazy_mmu() and xen_leave_lazy_mmu() in order
> > not to hurt all other lazy mmu mode users.
> >
> > Fixes: 291b3abed657 ("x86/xen: use lazy_mmu_state when context-switching")
> > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
>
> I have ran several test iterations with this patch (on top of 7.0.4) and
> it seems to fix the issue. So,
>
> Tested-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

Thanks for testing.

> I did run some tests also with 291b3abed657 reverted (instead of this
> patch), and that seems to work too, but I didn't run enough of
> iterations to be 100% sure. Would it be helpful to that that further
> too?

I do prefer my variant, as it is on my preferred path to get rid of the
Xen-private lazy mode tracking.

So in my personal opinion you don't need to continue this test.


Juergen

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