On Mon,  8 Sep 2025 08:39:26 +0100 Kevin Brodsky [off-list ref] wrote:

arch_{enter,leave}_lazy_mmu_mode() currently have a stateless API
(taking and returning no value). This is proving problematic in
situations where leave() needs to restore some context back to its
original state (before enter() was called). In particular, this
makes it difficult to support the nesting of lazy_mmu sections -
leave() does not know whether the matching enter() call occurred
while lazy_mmu was already enabled, and whether to disable it or
not.

This patch gives all architectures the chance to store local state
while inside a lazy_mmu section by making enter() return some value,
storing it in a local variable, and having leave() take that value.
That value is typed lazy_mmu_state_t - each architecture defining
__HAVE_ARCH_ENTER_LAZY_MMU_MODE is free to define it as it sees fit.
For now we define it as int everywhere, which is sufficient to
support nesting.

The diff is unfortunately rather large as all the API changes need
to be done atomically. Main parts:

This has a build error:

  CC      arch/x86/kernel/asm-offsets.s
In file included from ./arch/x86/include/asm/irqflags.h:102,
                 from ./include/linux/irqflags.h:18,
                 from ./include/linux/spinlock.h:59,
                 from ./include/linux/swait.h:7,
                 from ./include/linux/completion.h:12,
                 from ./include/linux/crypto.h:15,
                 from arch/x86/kernel/asm-offsets.c:9:
./arch/x86/include/asm/paravirt.h: In function 'arch_enter_lazy_mmu_mode':
./arch/x86/include/asm/paravirt.h:534:16: error: 'LAZY_MMU_DEFAULT' undeclared (first use in this function)
  534 |         return LAZY_MMU_DEFAULT;
      |                ^~~~~~~~~~~~~~~~
./arch/x86/include/asm/paravirt.h:534:16: note: each undeclared identifier is re

which gets fixed up later in the series.