[Xen-devel] [PATCH] mm/x86: Patch out arch_flush_lazy_mmu_mode() when running on bare metal

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

Invoking arch_flush_lazy_mmu_mode() results in calls to
preempt_enable()/disable() which may have performance impact.

Since lazy MMU is not used on bare metal we can patch away
arch_flush_lazy_mmu_mode() so that it is never called in such
environment.

Signed-off-by: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
---
 arch/x86/include/asm/paravirt.h       |  5 ++++-
 arch/x86/include/asm/paravirt_types.h |  2 ++
 arch/x86/kernel/paravirt.c            | 25 +++++++++++++------------
 arch/x86/lguest/boot.c                |  1 +
 arch/x86/xen/mmu.c                    |  1 +
 5 files changed, 21 insertions(+), 13 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 5edd174..7361e47 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -703,7 +703,10 @@ static inline void arch_leave_lazy_mmu_mode(void)
        PVOP_VCALL0(pv_mmu_ops.lazy_mode.leave);
 }
 
-void arch_flush_lazy_mmu_mode(void);
+static inline void arch_flush_lazy_mmu_mode(void)
+{
+       PVOP_VCALL0(pv_mmu_ops.lazy_mode.flush);
+}
 
 static inline void __set_fixmap(unsigned /* enum fixed_addresses */ idx,
                                phys_addr_t phys, pgprot_t flags)
diff --git a/arch/x86/include/asm/paravirt_types.h 
b/arch/x86/include/asm/paravirt_types.h
index 142236e..b3b0ec1 100644
--- a/arch/x86/include/asm/paravirt_types.h
+++ b/arch/x86/include/asm/paravirt_types.h
@@ -91,6 +91,7 @@ struct pv_lazy_ops {
        /* Set deferred update mode, used for batching operations. */
        void (*enter)(void);
        void (*leave)(void);
+       void (*flush)(void);
 };
 
 struct pv_time_ops {
@@ -679,6 +680,7 @@ void paravirt_end_context_switch(struct task_struct *next);
 
 void paravirt_enter_lazy_mmu(void);
 void paravirt_leave_lazy_mmu(void);
+void paravirt_flush_lazy_mmu(void);
 
 void _paravirt_nop(void);
 u32 _paravirt_ident_32(u32);
diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
index 17fff18..8bfb335 100644
--- a/arch/x86/kernel/paravirt.c
+++ b/arch/x86/kernel/paravirt.c
@@ -263,6 +263,18 @@ void paravirt_leave_lazy_mmu(void)
        leave_lazy(PARAVIRT_LAZY_MMU);
 }
 
+void paravirt_flush_lazy_mmu(void)
+{
+       preempt_disable();
+
+       if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
+               arch_leave_lazy_mmu_mode();
+               arch_enter_lazy_mmu_mode();
+       }
+
+       preempt_enable();
+}
+
 void paravirt_start_context_switch(struct task_struct *prev)
 {
        BUG_ON(preemptible());
@@ -292,18 +304,6 @@ enum paravirt_lazy_mode paravirt_get_lazy_mode(void)
        return this_cpu_read(paravirt_lazy_mode);
 }
 
-void arch_flush_lazy_mmu_mode(void)
-{
-       preempt_disable();
-
-       if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_MMU) {
-               arch_leave_lazy_mmu_mode();
-               arch_enter_lazy_mmu_mode();
-       }
-
-       preempt_enable();
-}
-
 struct pv_info pv_info = {
        .name = "bare hardware",
        .paravirt_enabled = 0,
@@ -475,6 +475,7 @@ struct pv_mmu_ops pv_mmu_ops = {
        .lazy_mode = {
                .enter = paravirt_nop,
                .leave = paravirt_nop,
+               .flush = paravirt_nop,
        },
 
        .set_fixmap = native_set_fixmap,
diff --git a/arch/x86/lguest/boot.c b/arch/x86/lguest/boot.c
index 1cbd89c..7114c63 100644
--- a/arch/x86/lguest/boot.c
+++ b/arch/x86/lguest/boot.c
@@ -1334,6 +1334,7 @@ __init void lguest_init(void)
        pv_mmu_ops.read_cr3 = lguest_read_cr3;
        pv_mmu_ops.lazy_mode.enter = paravirt_enter_lazy_mmu;
        pv_mmu_ops.lazy_mode.leave = lguest_leave_lazy_mmu_mode;
+       pv_mmu_ops.lazy_mode.flush = paravirt_flush_lazy_mmu;
        pv_mmu_ops.pte_update = lguest_pte_update;
        pv_mmu_ops.pte_update_defer = lguest_pte_update;
 
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index e8e3493..f4f4105 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -2197,6 +2197,7 @@ static const struct pv_mmu_ops xen_mmu_ops __initconst = {
        .lazy_mode = {
                .enter = paravirt_enter_lazy_mmu,
                .leave = xen_leave_lazy_mmu,
+               .flush = paravirt_flush_lazy_mmu,
        },
 
        .set_fixmap = xen_set_fixmap,
-- 
1.8.1.2


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