[Xen-devel] [PATCH 39/60] x86: optimize loading of GDT at context switch

[Juergen Gross <jgross@xxxxxxxx>]

Instead of dynamically decide whether the previous vcpu was using full
or default GDT just add a percpu variable for that purpose. This at
once removes the need for testing vcpu_ids to differ twice.

Cache the need_full_gdt(nd) value in a local variable.

Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
RFC V2: new patch (split from previous one)
V1: init percpu flag at cpu startup
    rename variable (Jan Beulich)
---
 xen/arch/x86/cpu/common.c  |  3 +++
 xen/arch/x86/domain.c      | 16 +++++++++++-----
 xen/include/asm-x86/desc.h |  1 +
 3 files changed, 15 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 33f5d32557..8b90356fe5 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -49,6 +49,8 @@ unsigned int vaddr_bits __read_mostly = VADDR_BITS;
 static unsigned int cleared_caps[NCAPINTS];
 static unsigned int forced_caps[NCAPINTS];
 
+DEFINE_PER_CPU(bool, full_gdt_loaded);
+
 void __init setup_clear_cpu_cap(unsigned int cap)
 {
        const uint32_t *dfs;
@@ -745,6 +747,7 @@ void load_system_tables(void)
                offsetof(struct tss_struct, __cacheline_filler) - 1,
                SYS_DESC_tss_busy);
 
+        per_cpu(full_gdt_loaded, cpu) = false;
        lgdt(&gdtr);
        lidt(&idtr);
        ltr(TSS_ENTRY << 3);
diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index adc06154ee..98d2939daf 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -1645,6 +1645,8 @@ static inline void load_full_gdt(const struct vcpu *v, 
unsigned int cpu)
     };
 
     lgdt(&gdt_desc);
+
+    per_cpu(full_gdt_loaded, cpu) = true;
 }
 
 static inline void load_default_gdt(const seg_desc_t *gdt, unsigned int cpu)
@@ -1655,6 +1657,8 @@ static inline void load_default_gdt(const seg_desc_t 
*gdt, unsigned int cpu)
     };
 
     lgdt(&gdt_desc);
+
+    per_cpu(full_gdt_loaded, cpu) = false;
 }
 
 static void __context_switch(void)
@@ -1665,6 +1669,7 @@ static void __context_switch(void)
     struct vcpu          *n = current;
     struct domain        *pd = p->domain, *nd = n->domain;
     seg_desc_t           *gdt;
+    bool                  full_gdt;
 
     ASSERT(p != n);
     ASSERT(!vcpu_cpu_dirty(n));
@@ -1707,11 +1712,13 @@ static void __context_switch(void)
     gdt = !is_pv_32bit_domain(nd) ? per_cpu(gdt_table, cpu) :
                                     per_cpu(compat_gdt_table, cpu);
 
-    if ( need_full_gdt(nd) )
+    full_gdt = need_full_gdt(nd);
+
+    if ( full_gdt )
         write_full_gdt_ptes(gdt, n);
 
-    if ( need_full_gdt(pd) &&
-         ((p->vcpu_id != n->vcpu_id) || !need_full_gdt(nd)) )
+    if ( per_cpu(full_gdt_loaded, cpu) &&
+         ((p->vcpu_id != n->vcpu_id) || !full_gdt) )
         load_default_gdt(gdt, cpu);
 
     write_ptbase(n);
@@ -1723,8 +1730,7 @@ static void __context_switch(void)
         svm_load_segs(0, 0, 0, 0, 0, 0, 0);
 #endif
 
-    if ( need_full_gdt(nd) &&
-         ((p->vcpu_id != n->vcpu_id) || !need_full_gdt(pd)) )
+    if ( full_gdt && !per_cpu(full_gdt_loaded, cpu) )
         load_full_gdt(n, cpu);
 
     if ( pd != nd )
diff --git a/xen/include/asm-x86/desc.h b/xen/include/asm-x86/desc.h
index 85e83bcefb..ff9ac5f15d 100644
--- a/xen/include/asm-x86/desc.h
+++ b/xen/include/asm-x86/desc.h
@@ -208,6 +208,7 @@ extern seg_desc_t boot_cpu_gdt_table[];
 DECLARE_PER_CPU(seg_desc_t *, gdt_table);
 extern seg_desc_t boot_cpu_compat_gdt_table[];
 DECLARE_PER_CPU(seg_desc_t *, compat_gdt_table);
+DECLARE_PER_CPU(bool, full_gdt_loaded);
 
 extern void load_TR(void);
 
-- 
2.16.4


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