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

[Jürgen Groß]

On 08.05.26 22:54, Kevin Brodsky wrote:
> On 08/05/2026 16:39, 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.
>
> I think this is a correct description of the issue, i.e. potentially we
> have arch_enter_lazy_mmu_mode() called twice *sequentially*. Therefore I
> don't think that disabling preemption inside arch_enter_lazy_mmu_mode()
> is enough - we have a problem with preemption occurring inside
> lazy_mmu_mode_enable() generally, not necessarily inside
> arch_enter_lazy_mmu_mode().
>
> Preemption shouldn't matter if commit 291b3abed657 is reverted. AFAICT
> this is the only easy fix.
The description wasn't really complete, I think.

The double call will only be possible if arch_end_context_switch() is
calling arch_enter_lazy_mmu_mode(), and this is happening for Xen PV only.
arch_end_context_switch() is a nop for all other cases.

So this can be handled completely internal of Xen (otherwise a revert of
291b3abed657 wouldn't help), and it is easy to do so as my patch is
showing.

As said, I'd like to get rid of the extra tracking by Xen regarding lazy mode.


Juergen

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