xen: Add support for preemption

Add Xen support for preemption.  This is mostly a cleanup of existing
preempt_enable/disable calls, or just comments to explain the current
usage.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Signed-off-by: Chris Wright <chrisw@sous-sol.org>

5 files changed, 76 insertions, 42 deletions

diff --git a/arch/i386/xen/Kconfig b/arch/i386/xen/Kconfig
index b7697ff22361..9df99e1885a4 100644
--- a/arch/i386/xen/Kconfig
+++ b/arch/i386/xen/Kconfig
@@ -4,7 +4,7 @@
 
 config XEN
         bool "Enable support for Xen hypervisor"
-        depends on PARAVIRT && X86_CMPXCHG && X86_TSC && !(PREEMPT || NEED_MULTIPLE_NODES)
+        depends on PARAVIRT && X86_CMPXCHG && X86_TSC && !NEED_MULTIPLE_NODES
         help
           This is the Linux Xen port.  Enabling this will allow the
           kernel to boot in a paravirtualized environment under the
diff --git a/arch/i386/xen/enlighten.c b/arch/i386/xen/enlighten.c
index de62d66e0893..a1124b7f1d14 100644
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -15,6 +15,7 @@
 #include <linux/init.h>
 #include <linux/smp.h>
 #include <linux/preempt.h>
+#include <linux/hardirq.h>
 #include <linux/percpu.h>
 #include <linux/delay.h>
 #include <linux/start_kernel.h>
@@ -108,11 +109,10 @@ static unsigned long xen_save_fl(void)
         struct vcpu_info *vcpu;
         unsigned long flags;
 
-        preempt_disable();
         vcpu = x86_read_percpu(xen_vcpu);
+
         /* flag has opposite sense of mask */
         flags = !vcpu->evtchn_upcall_mask;
-        preempt_enable();
 
         /* convert to IF type flag
            -0 -> 0x00000000
@@ -125,32 +125,35 @@ static void xen_restore_fl(unsigned long flags)
 {
         struct vcpu_info *vcpu;
 
-        preempt_disable();
-
         /* convert from IF type flag */
         flags = !(flags & X86_EFLAGS_IF);
+
+        /* There's a one instruction preempt window here.  We need to
+           make sure we're don't switch CPUs between getting the vcpu
+           pointer and updating the mask. */
+        preempt_disable();
         vcpu = x86_read_percpu(xen_vcpu);
         vcpu->evtchn_upcall_mask = flags;
+        preempt_enable_no_resched();
 
-        if (flags == 0) {
-                /* Unmask then check (avoid races).  We're only protecting
-                   against updates by this CPU, so there's no need for
-                   anything stronger. */
-                barrier();
+        /* Doesn't matter if we get preempted here, because any
+           pending event will get dealt with anyway. */
 
+        if (flags == 0) {
+                preempt_check_resched();
+                barrier(); /* unmask then check (avoid races) */
                 if (unlikely(vcpu->evtchn_upcall_pending))
                         force_evtchn_callback();
-                preempt_enable();
-        } else
-                preempt_enable_no_resched();
+        }
 }
 
 static void xen_irq_disable(void)
 {
-        struct vcpu_info *vcpu;
+        /* There's a one instruction preempt window here.  We need to
+           make sure we're don't switch CPUs between getting the vcpu
+           pointer and updating the mask. */
         preempt_disable();
-        vcpu = x86_read_percpu(xen_vcpu);
-        vcpu->evtchn_upcall_mask = 1;
+        x86_read_percpu(xen_vcpu)->evtchn_upcall_mask = 1;
         preempt_enable_no_resched();
 }
 
@@ -158,18 +161,20 @@ static void xen_irq_enable(void)
 {
         struct vcpu_info *vcpu;
 
+        /* There's a one instruction preempt window here.  We need to
+           make sure we're don't switch CPUs between getting the vcpu
+           pointer and updating the mask. */
         preempt_disable();
         vcpu = x86_read_percpu(xen_vcpu);
         vcpu->evtchn_upcall_mask = 0;
+        preempt_enable_no_resched();
 
-        /* Unmask then check (avoid races).  We're only protecting
-           against updates by this CPU, so there's no need for
-           anything stronger. */
-        barrier();
+        /* Doesn't matter if we get preempted here, because any
+           pending event will get dealt with anyway. */
 
+        barrier(); /* unmask then check (avoid races) */
         if (unlikely(vcpu->evtchn_upcall_pending))
                 force_evtchn_callback();
-        preempt_enable();
 }
 
 static void xen_safe_halt(void)
@@ -189,6 +194,8 @@ static void xen_halt(void)
 
 static void xen_set_lazy_mode(enum paravirt_lazy_mode mode)
 {
+        BUG_ON(preemptible());
+
         switch (mode) {
         case PARAVIRT_LAZY_NONE:
                 BUG_ON(x86_read_percpu(xen_lazy_mode) == PARAVIRT_LAZY_NONE);
@@ -293,9 +300,13 @@ static void xen_write_ldt_entry(struct desc_struct *dt, int entrynum,
         xmaddr_t mach_lp = virt_to_machine(lp);
         u64 entry = (u64)high << 32 | low;
 
+        preempt_disable();
+
         xen_mc_flush();
         if (HYPERVISOR_update_descriptor(mach_lp.maddr, entry))
                 BUG();
+
+        preempt_enable();
 }
 
 static int cvt_gate_to_trap(int vector, u32 low, u32 high,
@@ -328,11 +339,13 @@ static DEFINE_PER_CPU(struct Xgt_desc_struct, idt_desc);
 static void xen_write_idt_entry(struct desc_struct *dt, int entrynum,
                                 u32 low, u32 high)
 {
-
-        int cpu = smp_processor_id();
         unsigned long p = (unsigned long)&dt[entrynum];
-        unsigned long start = per_cpu(idt_desc, cpu).address;
-        unsigned long end = start + per_cpu(idt_desc, cpu).size + 1;
+        unsigned long start, end;
+
+        preempt_disable();
+
+        start = __get_cpu_var(idt_desc).address;
+        end = start + __get_cpu_var(idt_desc).size + 1;
 
         xen_mc_flush();
 
@@ -347,6 +360,8 @@ static void xen_write_idt_entry(struct desc_struct *dt, int entrynum,
                         if (HYPERVISOR_set_trap_table(info))
                                 BUG();
         }
+
+        preempt_enable();
 }
 
 static void xen_convert_trap_info(const struct Xgt_desc_struct *desc,
@@ -368,11 +383,9 @@ static void xen_convert_trap_info(const struct Xgt_desc_struct *desc,
 
 void xen_copy_trap_info(struct trap_info *traps)
 {
-        const struct Xgt_desc_struct *desc = &get_cpu_var(idt_desc);
+        const struct Xgt_desc_struct *desc = &__get_cpu_var(idt_desc);
 
         xen_convert_trap_info(desc, traps);
-
-        put_cpu_var(idt_desc);
 }
 
 /* Load a new IDT into Xen.  In principle this can be per-CPU, so we
@@ -382,12 +395,11 @@ static void xen_load_idt(const struct Xgt_desc_struct *desc)
 {
         static DEFINE_SPINLOCK(lock);
         static struct trap_info traps[257];
-        int cpu = smp_processor_id();
-
-        per_cpu(idt_desc, cpu) = *desc;
 
         spin_lock(&lock);
 
+        __get_cpu_var(idt_desc) = *desc;
+
         xen_convert_trap_info(desc, traps);
 
         xen_mc_flush();
@@ -402,6 +414,8 @@ static void xen_load_idt(const struct Xgt_desc_struct *desc)
 static void xen_write_gdt_entry(struct desc_struct *dt, int entry,
                                 u32 low, u32 high)
 {
+        preempt_disable();
+
         switch ((high >> 8) & 0xff) {
         case DESCTYPE_LDT:
         case DESCTYPE_TSS:
@@ -418,10 +432,12 @@ static void xen_write_gdt_entry(struct desc_struct *dt, int entry,
         }
 
         }
+
+        preempt_enable();
 }
 
 static void xen_load_esp0(struct tss_struct *tss,
-                                   struct thread_struct *thread)
+                          struct thread_struct *thread)
 {
         struct multicall_space mcs = xen_mc_entry(0);
         MULTI_stack_switch(mcs.mc, __KERNEL_DS, thread->esp0);
@@ -525,6 +541,8 @@ static unsigned long xen_read_cr3(void)
 
 static void xen_write_cr3(unsigned long cr3)
 {
+        BUG_ON(preemptible());
+
         if (cr3 == x86_read_percpu(xen_cr3)) {
                 /* just a simple tlb flush */
                 xen_flush_tlb();
diff --git a/arch/i386/xen/mmu.c b/arch/i386/xen/mmu.c
index bc49ef846203..f431cf14e644 100644
--- a/arch/i386/xen/mmu.c
+++ b/arch/i386/xen/mmu.c
@@ -38,6 +38,7 @@
  *
  * Jeremy Fitzhardinge <jeremy@xensource.com>, XenSource Inc, 2007
  */
+#include <linux/sched.h>
 #include <linux/highmem.h>
 #include <linux/bug.h>
 #include <linux/sched.h>
@@ -531,5 +532,7 @@ void xen_exit_mmap(struct mm_struct *mm)
         drop_mm_ref(mm);
         put_cpu();
 
+        spin_lock(&mm->page_table_lock);
         xen_pgd_unpin(mm->pgd);
+        spin_unlock(&mm->page_table_lock);
 }
diff --git a/arch/i386/xen/multicalls.c b/arch/i386/xen/multicalls.c
index 869f9833f08f..d4015a9ed46c 100644
--- a/arch/i386/xen/multicalls.c
+++ b/arch/i386/xen/multicalls.c
@@ -20,6 +20,7 @@
  * Jeremy Fitzhardinge <jeremy@xensource.com>, XenSource Inc, 2007
  */
 #include <linux/percpu.h>
+#include <linux/hardirq.h>
 
 #include <asm/xen/hypercall.h>
 
@@ -39,10 +40,12 @@ DEFINE_PER_CPU(unsigned long, xen_mc_irq_flags);
 
 void xen_mc_flush(void)
 {
-        struct mc_buffer *b = &get_cpu_var(mc_buffer);
+        struct mc_buffer *b = &__get_cpu_var(mc_buffer);
         int ret = 0;
         unsigned long flags;
 
+        BUG_ON(preemptible());
+
         /* Disable interrupts in case someone comes in and queues
            something in the middle */
         local_irq_save(flags);
@@ -60,7 +63,6 @@ void xen_mc_flush(void)
         } else
                 BUG_ON(b->argidx != 0);
 
-        put_cpu_var(mc_buffer);
         local_irq_restore(flags);
 
         BUG_ON(ret);
@@ -68,10 +70,11 @@ void xen_mc_flush(void)
 
 struct multicall_space __xen_mc_entry(size_t args)
 {
-        struct mc_buffer *b = &get_cpu_var(mc_buffer);
+        struct mc_buffer *b = &__get_cpu_var(mc_buffer);
         struct multicall_space ret;
         unsigned argspace = (args + sizeof(u64) - 1) / sizeof(u64);
 
+        BUG_ON(preemptible());
         BUG_ON(argspace > MC_ARGS);
 
         if (b->mcidx == MC_BATCH ||
@@ -83,7 +86,5 @@ struct multicall_space __xen_mc_entry(size_t args)
         ret.args = &b->args[b->argidx];
         b->argidx += argspace;
 
-        put_cpu_var(mc_buffer);
-
         return ret;
 }
diff --git a/arch/i386/xen/time.c b/arch/i386/xen/time.c
index aeb04cf5dbf1..51fdabf1fd4d 100644
--- a/arch/i386/xen/time.c
+++ b/arch/i386/xen/time.c
@@ -88,7 +88,7 @@ static void get_runstate_snapshot(struct vcpu_runstate_info *res)
         u64 state_time;
         struct vcpu_runstate_info *state;
 
-        preempt_disable();
+        BUG_ON(preemptible());
 
         state = &__get_cpu_var(runstate);
 
@@ -103,8 +103,6 @@ static void get_runstate_snapshot(struct vcpu_runstate_info *res)
                 *res = *state;
                 barrier();
         } while (get64(&state->state_entry_time) != state_time);
-
-        preempt_enable();
 }
 
 static void setup_runstate_info(int cpu)
@@ -179,9 +177,19 @@ static void do_stolen_accounting(void)
 unsigned long long xen_sched_clock(void)
 {
         struct vcpu_runstate_info state;
-        cycle_t now = xen_clocksource_read();
+        cycle_t now;
+        u64 ret;
         s64 offset;
 
+        /*
+         * Ideally sched_clock should be called on a per-cpu basis
+         * anyway, so preempt should already be disabled, but that's
+         * not current practice at the moment.
+         */
+        preempt_disable();
+
+        now = xen_clocksource_read();
+
         get_runstate_snapshot(&state);
 
         WARN_ON(state.state != RUNSTATE_running);
@@ -190,9 +198,13 @@ unsigned long long xen_sched_clock(void)
         if (offset < 0)
                 offset = 0;
 
-        return state.time[RUNSTATE_blocked] +
+        ret = state.time[RUNSTATE_blocked] +
                 state.time[RUNSTATE_running] +
                 offset;
+
+        preempt_enable();
+
+        return ret;
 }