On 09.09.25 11:13, David Hildenbrand wrote:

On 08.09.25 09:39, Kevin Brodsky wrote:

quoted

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.

Signed-off-by: Kevin Brodsky <redacted>
---
  arch/x86/include/asm/paravirt.h       |  6 ++----
  arch/x86/include/asm/paravirt_types.h |  4 ++--
  arch/x86/xen/mmu_pv.c                 | 11 ++++++++---
  3 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 65a0d394fba1..4ecd3a6b1dea 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h

@@ -529,14 +529,12 @@ static inline void arch_end_context_switch(struct 

task_struct *next)
  #define  __HAVE_ARCH_ENTER_LAZY_MMU_MODE
  static inline lazy_mmu_state_t arch_enter_lazy_mmu_mode(void)
  {
-    PVOP_VCALL0(mmu.lazy_mode.enter);
-
-    return LAZY_MMU_DEFAULT;
+    return PVOP_CALL0(lazy_mmu_state_t, mmu.lazy_mode.enter);
  }
  static inline void arch_leave_lazy_mmu_mode(lazy_mmu_state_t state)
  {
-    PVOP_VCALL0(mmu.lazy_mode.leave);
+    PVOP_VCALL1(mmu.lazy_mode.leave, state);
  }
  static inline void arch_flush_lazy_mmu_mode(void)

diff --git a/arch/x86/include/asm/paravirt_types.h b/arch/x86/include/asm/ 

paravirt_types.h
index bc1af86868a3..b7c567ccbf32 100644

--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h

@@ -45,8 +45,8 @@ typedef int lazy_mmu_state_t;

  struct pv_lazy_ops {
      /* Set deferred update mode, used for batching operations. */
-    void (*enter)(void);
-    void (*leave)(void);
+    lazy_mmu_state_t (*enter)(void);
+    void (*leave)(lazy_mmu_state_t);
      void (*flush)(void);
  } __no_randomize_layout;
  #endif

diff --git a/arch/x86/xen/mmu_pv.c b/arch/x86/xen/mmu_pv.c
index 2039d5132ca3..6e5390ff06a5 100644
--- a/arch/x86/xen/mmu_pv.c
+++ b/arch/x86/xen/mmu_pv.c

@@ -2130,9 +2130,13 @@ static void xen_set_fixmap(unsigned idx, phys_addr_t 

phys, pgprot_t prot)
  #endif
  }
-static void xen_enter_lazy_mmu(void)
+static lazy_mmu_state_t xen_enter_lazy_mmu(void)
  {
+    if (this_cpu_read(xen_lazy_mode) == XEN_LAZY_MMU)
+        return LAZY_MMU_NESTED;
+

You mention above "preemption-safe manner" above, so I am wondering,
what if we get preempted immediately after doing the this_cpu_read() and get 
scheduled on another CPU?

This should still be correct: preemption needs a context switch to happen,
so xen_start_context_switch() and xen_end_context_switch() are involved.
Those are dealing with this problem by doing the right thing in the old
and the new context.


Juergen