On Thu, Sep 04, 2025 at 01:57:31PM +0100, Kevin Brodsky 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:

* Changing the prototypes of arch_{enter,leave}_lazy_mmu_mode()
  in generic and arch code, and introducing lazy_mmu_state_t.

* Introducing LAZY_MMU_{DEFAULT,NESTED} for future support of
  nesting. enter() always returns LAZY_MMU_DEFAULT for now.
  (linux/mm_types.h is not the most natural location for defining
  those constants, but there is no other obvious header that is
  accessible where arch's implement the helpers.)

* Changing all lazy_mmu sections to introduce a lazy_mmu_state
  local variable, having enter() set it and leave() take it. Most of
  these changes were generated using the Coccinelle script below.

@@
@@
{
+ lazy_mmu_state_t lazy_mmu_state;
...
- arch_enter_lazy_mmu_mode();
+ lazy_mmu_state = arch_enter_lazy_mmu_mode();
...
- arch_leave_lazy_mmu_mode();
+ arch_leave_lazy_mmu_mode(lazy_mmu_state);
...
}

Note: it is difficult to provide a default definition of
lazy_mmu_state_t for architectures implementing lazy_mmu, because
that definition would need to be available in
arch/x86/include/asm/paravirt_types.h and adding a new generic
 #include there is very tricky due to the existing header soup.

Signed-off-by: Kevin Brodsky <redacted>

Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>

-- 
Sincerely yours,
Mike.