[PATCH 4/7] x86/xen: support nested lazy_mmu sections (again)
From: Kevin Brodsky <hidden>
Date: 2025-09-04 12:58:40
Also in:
linux-arm-kernel, linux-mm, lkml, sparclinux, xen-devel
Subsystem:
paravirt_ops interface, the rest, x86 architecture (32-bit and 64-bit), xen hypervisor x86 · Maintainers:
Juergen Gross, Linus Torvalds, Thomas Gleixner, Ingo Molnar, Borislav Petkov, Dave Hansen
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; + 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