x86/xen: support nested lazy_mmu sections (again)

x86/xen: support nested lazy_mmu sections (again)

Commit 49147beb0ccb ("x86/xen: allow nesting of same lazy mode")
originally introduced support for nested lazy sections (LAZY_MMU and
LAZY_CPU).  It later got reverted by commit c36549ff8d84 as its
implementation turned out to be intolerant to preemption.

Now that the lazy_mmu API allows enter() to pass through a state to the
matching leave() call, we can support nesting again for the LAZY_MMU mode
in a preemption-safe manner.  If xen_enter_lazy_mmu() is called inside an
active lazy_mmu section, xen_lazy_mode will already be set to XEN_LAZY_MMU
and we can then return LAZY_MMU_NESTED to instruct the matching
xen_leave_lazy_mmu() call to leave xen_lazy_mode unchanged.

The only effect of this patch is to ensure that xen_lazy_mode remains set
to XEN_LAZY_MMU until the outermost lazy_mmu section ends.
xen_leave_lazy_mmu() still calls xen_mc_flush() unconditionally.

Link: https://lkml.kernel.org/r/20250908073931.4159362-5-kevin.brodsky@arm.com
Signed-off-by: Kevin Brodsky <kevin.brodsky@arm.com>
Cc: Alexander Gordeev <agordeev@linux.ibm.com>
Cc: Andreas Larsson <andreas@gaisler.com>
Cc: Borislav Betkov <bp@alien8.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Christophe Leroy <christophe.leroy@csgroup.eu>
Cc: David Hildenbrand <david@redhat.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jann Horn <jannh@google.com>
Cc: Juegren Gross <jgross@suse.com>
Cc: levi.yun <yeoreum.yun@arm.com>
Cc: Liam Howlett <liam.howlett@oracle.com>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Mike Rapoport (Microsoft) <rppt@kernel.org>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Thomas Gleinxer <tglx@linutronix.de>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>