[PATCH v2 4/7] x86/xen: support nested lazy_mmu sections (again)

Kevin Brodsky
2025-09-08T07:40:54Z

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 <kevin.brodsky@arm.com>
---
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;
+
enter_lazy(XEN_LAZY_MMU);
+ return LAZY_MMU_DEFAULT;
}

static void xen_flush_lazy_mmu(void)
@@ -2167,11 +2171,12 @@ static void __init xen_post_allocator_init(void)
pv_ops.mmu.write_cr3 = &xen_write_cr3;
}

-static void xen_leave_lazy_mmu(void)
+static void xen_leave_lazy_mmu(lazy_mmu_state_t state)
{
preempt_disable();
xen_mc_flush();
- leave_lazy(XEN_LAZY_MMU);
+ if (state != LAZY_MMU_NESTED)
+ leave_lazy(XEN_LAZY_MMU);
preempt_enable();
}

--
2.47.0