[RFC,41/49] x86: make loading of GDT at context switch more modular

Commit Message

Juergen Gross March 29, 2019, 3:09 p.m. UTC

In preparation for core scheduling carve out the GDT related
functionality (writing GDT related PTEs, loading default of full GDT)
into sub-functions.

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 <[e-mail]>
---
 xen/arch/x86/domain.c | 71 +++++++++++++++++++++++++++++++++------------------
 1 file changed, 46 insertions(+), 25 deletions(-)

Patch

diff --git a/xen/arch/x86/domain.c b/xen/arch/x86/domain.c
index 7daba4fb91..5e764d8a54 100644
--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -73,6 +73,8 @@ 
 
 DEFINE_PER_CPU(struct vcpu *, curr_vcpu);
 
+static DEFINE_PER_CPU(bool, full_gdt_loaded);
+
 static void default_idle(void);
 void (*pm_idle) (void) __read_mostly = default_idle;
 void (*dead_idle) (void) __read_mostly = default_dead_idle;
@@ -1614,6 +1616,41 @@  static inline bool need_full_gdt(const struct domain *d)
     return is_pv_domain(d) && !is_idle_domain(d);
 }
 
+static inline void write_full_gdt_ptes(seg_desc_t *gdt, struct vcpu *v)
+{
+    unsigned long mfn = virt_to_mfn(gdt);
+    l1_pgentry_t *pl1e = pv_gdt_ptes(v);
+    unsigned int i;
+
+    for ( i = 0; i < NR_RESERVED_GDT_PAGES; i++ )
+        l1e_write(pl1e + FIRST_RESERVED_GDT_PAGE + i,
+                  l1e_from_pfn(mfn + i, __PAGE_HYPERVISOR_RW));
+}
+
+static inline void load_full_gdt(struct vcpu *v, unsigned int cpu)
+{
+    struct desc_ptr gdt_desc;
+
+    gdt_desc.limit = LAST_RESERVED_GDT_BYTE;
+    gdt_desc.base = GDT_VIRT_START(v);
+
+    lgdt(&gdt_desc);
+
+    per_cpu(full_gdt_loaded, cpu) = true;
+}
+
+static inline void load_default_gdt(seg_desc_t *gdt, unsigned int cpu)
+{
+    struct desc_ptr gdt_desc;
+
+    gdt_desc.limit = LAST_RESERVED_GDT_BYTE;
+    gdt_desc.base  = (unsigned long)(gdt - FIRST_RESERVED_GDT_ENTRY);
+
+    lgdt(&gdt_desc);
+
+    per_cpu(full_gdt_loaded, cpu) = false;
+}
+
 static void __context_switch(void)
 {
     struct cpu_user_regs *stack_regs = guest_cpu_user_regs();
@@ -1622,7 +1659,7 @@  static void __context_switch(void)
     struct vcpu          *n = current;
     struct domain        *pd = p->domain, *nd = n->domain;
     seg_desc_t           *gdt;
-    struct desc_ptr       gdt_desc;
+    bool                  need_full_gdt_n;
 
     ASSERT(p != n);
     ASSERT(!vcpu_cpu_dirty(n));
@@ -1664,25 +1701,15 @@  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) )
-    {
-        unsigned long mfn = virt_to_mfn(gdt);
-        l1_pgentry_t *pl1e = pv_gdt_ptes(n);
-        unsigned int i;
 
-        for ( i = 0; i < NR_RESERVED_GDT_PAGES; i++ )
-            l1e_write(pl1e + FIRST_RESERVED_GDT_PAGE + i,
-                      l1e_from_pfn(mfn + i, __PAGE_HYPERVISOR_RW));
-    }
+    need_full_gdt_n = need_full_gdt(nd);
 
-    if ( need_full_gdt(pd) &&
-         ((p->vcpu_id != n->vcpu_id) || !need_full_gdt(nd)) )
-    {
-        gdt_desc.limit = LAST_RESERVED_GDT_BYTE;
-        gdt_desc.base  = (unsigned long)(gdt - FIRST_RESERVED_GDT_ENTRY);
+    if ( need_full_gdt_n )
+        write_full_gdt_ptes(gdt, n);
 
-        lgdt(&gdt_desc);
-    }
+    if ( per_cpu(full_gdt_loaded, cpu) &&
+         ((p->vcpu_id != n->vcpu_id) || !need_full_gdt_n) )
+        load_default_gdt(gdt, cpu);
 
     write_ptbase(n);
 
@@ -1693,14 +1720,8 @@  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)) )
-    {
-        gdt_desc.limit = LAST_RESERVED_GDT_BYTE;
-        gdt_desc.base = GDT_VIRT_START(n);
-
-        lgdt(&gdt_desc);
-    }
+    if ( need_full_gdt_n && !per_cpu(full_gdt_loaded, cpu) )
+        load_full_gdt(n, cpu);
 
     if ( pd != nd )
         cpumask_clear_cpu(cpu, pd->dirty_cpumask);