Re: [PATCH v1 4/4] Revert "x86/xen: allow nesting of same lazy mode"

[David Hildenbrand <david@redhat.com>]

On 02.03.25 15:55, Ryan Roberts wrote:
> Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode") was
> added as a solution for a core-mm code change where
> arch_[enter|leave]_lazy_mmu_mode() started to be called in a nested
> manner; see commit bcc6cc832573 ("mm: add default definition of
> set_ptes()").
> 
> However, now that we have fixed the API to avoid nesting, we no longer
> need this capability in the x86 implementation.
> 
> Additionally, from code review, I don't believe the fix was ever robust
> in the case of preemption occurring while in the nested lazy mode. The
> implementation usually deals with preemption by calling
> arch_leave_lazy_mmu_mode() from xen_start_context_switch() for the
> outgoing task if we are in the lazy mmu mode. Then in
> xen_end_context_switch(), it restarts the lazy mode by calling
> arch_enter_lazy_mmu_mode() for an incoming task that was in the lazy
> mode when it was switched out. But arch_leave_lazy_mmu_mode() will only
> unwind a single level of nesting. If we are in the double nest, then
> it's not fully unwound and per-cpu variables are left in a bad state.
> 
> So the correct solution is to remove the possibility of nesting from the
> higher level (which has now been done) and remove this x86-specific
> solution.
> 
> Fixes: 49147beb0ccb ("x86/xen: allow nesting of same lazy mode")

Does this patch here deserve this tag? IIUC, it's rather a cleanup now 
that it was properly fixed elsewhere.

> Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers,

David / dhildenb