[Xen-devel] [PATCH RFC 22/44] x86/mm: Track whether the current cr3 has a short or extended directmap

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

This will be used to remove the mapcache override/current vcpu mechanism when
reworking map_domain_page() to be safe in the middle of context switches.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/mm.c        | 11 +++++++++++
 xen/arch/x86/setup.c     |  2 ++
 xen/common/efi/runtime.c |  3 +++
 xen/include/asm-x86/mm.h | 11 +++++++++++
 4 files changed, 27 insertions(+)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 57b3e25..7c08807 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -499,6 +499,7 @@ void make_cr3(struct vcpu *v, mfn_t mfn)
 }
 
 DEFINE_PER_CPU(unsigned long, curr_ptbase);
+DEFINE_PER_CPU(bool, curr_extended_directmap);
 
 void do_write_ptbase(struct vcpu *v, bool tlb_maintenance)
 {
@@ -506,6 +507,8 @@ void do_write_ptbase(struct vcpu *v, bool tlb_maintenance)
     unsigned int cpu = smp_processor_id();
     unsigned long *this_curr_ptbase = &per_cpu(curr_ptbase, cpu);
     l4_pgentry_t percpu_mappings = per_cpu(percpu_mappings, cpu);
+    bool *this_extd_directmap = &per_cpu(curr_extended_directmap, cpu);
+    bool new_extd_directmap = paging_mode_external(v->domain);
     l4_pgentry_t *new_l4t;
     struct page_info *new_pg;
 
@@ -529,11 +532,19 @@ void do_write_ptbase(struct vcpu *v, bool tlb_maintenance)
     new_l4t[l4_table_offset(PERCPU_LINEAR_START)] = percpu_mappings;
     barrier();
 
+    /* If the new cr3 has a short directmap, report so before switching... */
+    if ( !new_extd_directmap )
+        *this_extd_directmap = new_extd_directmap;
+
     if ( tlb_maintenance )
         write_cr3(new_cr3);
     else
         asm volatile ( "mov %0, %%cr3" :: "r" (new_cr3) : "memory" );
 
+    /* ... else report afterwards. */
+    if ( new_extd_directmap )
+        *this_extd_directmap = new_extd_directmap;
+
     /* Mark the old cr3 as no longer in use. */
     if ( new_cr3 != *this_curr_ptbase )
     {
diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index 662c383..80efef0 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -270,6 +270,7 @@ void early_switch_to_idle(bool bsp)
                    : "memory" );
 
     per_cpu(curr_ptbase, cpu) = v->arch.cr3;
+    per_cpu(curr_extended_directmap, cpu) = true;
 
     lidt(&idtr);
 
@@ -713,6 +714,7 @@ void __init noreturn __start_xen(unsigned long mbi_p)
     idle_vcpu[0] = current;
 
     percpu_init_areas();
+    per_cpu(curr_extended_directmap, 0) = true;
 
     init_idt_traps();
     {
diff --git a/xen/common/efi/runtime.c b/xen/common/efi/runtime.c
index 3dbc2e8..fe6d3af 100644
--- a/xen/common/efi/runtime.c
+++ b/xen/common/efi/runtime.c
@@ -112,6 +112,7 @@ struct efi_rs_state efi_rs_enter(void)
     }
 
     write_cr3(virt_to_maddr(efi_l4_pgtable));
+    this_cpu(curr_extended_directmap) = true;
 
     return state;
 }
@@ -120,6 +121,8 @@ void efi_rs_leave(struct efi_rs_state *state)
 {
     if ( !state->cr3 )
         return;
+
+    this_cpu(curr_extended_directmap) = paging_mode_external(current->domain);
     write_cr3(state->cr3);
     if ( is_pv_vcpu(current) && !is_idle_vcpu(current) )
     {
diff --git a/xen/include/asm-x86/mm.h b/xen/include/asm-x86/mm.h
index 64044c6..54b7499 100644
--- a/xen/include/asm-x86/mm.h
+++ b/xen/include/asm-x86/mm.h
@@ -567,6 +567,17 @@ void audit_domains(void);
 
 DECLARE_PER_CPU(unsigned long, curr_ptbase);
 
+/*
+ * Indicates whether the current %cr3 contains a short or extended directmap.
+ * Care needs to be taken when updating, as map_domain_page() is used in
+ * interrupt/exception context.  It is safe to indicate that the current %cr3
+ * is short when it is actually extended (in which case, map_domain_page()
+ * will use a mapping slot rather than refering to the directmap), but it is
+ * not safe to indicate the opposite (in which case, map_domain_page() will
+ * return a pointer into 64bit PV kernel address space).
+ */
+DECLARE_PER_CPU(bool, curr_extended_directmap);
+
 void make_cr3(struct vcpu *v, mfn_t mfn);
 void update_cr3(struct vcpu *v);
 int vcpu_destroy_pagetables(struct vcpu *);
-- 
2.1.4


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