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Subject[PATCH 12/25] 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>

---
arch/i386/xen/Kconfig | 2
arch/i386/xen/enlighten.c | 93 ++++++++++++++++++++++++++++----------------
arch/i386/xen/mmu.c | 4 +
arch/i386/xen/multicalls.c | 11 ++---
arch/i386/xen/time.c | 22 ++++++++--
5 files changed, 88 insertions(+), 44 deletions(-)
===================================================================
--- 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 && !PREEMPT
+ depends on PARAVIRT
default y
help
This is the Linux Xen port.
===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -2,6 +2,7 @@
#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>
@@ -92,11 +93,10 @@ 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
@@ -109,41 +109,56 @@ static void xen_restore_fl(unsigned long
{
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();
+
+ /* 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)
{
+ /* 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();
+ x86_read_percpu(xen_vcpu)->evtchn_upcall_mask = 1;
+ preempt_enable_no_resched();
+}
+
+static void xen_irq_enable(void)
+{
struct vcpu_info *vcpu;
- preempt_disable();
- vcpu = x86_read_percpu(xen_vcpu);
- vcpu->evtchn_upcall_mask = 1;
- preempt_enable_no_resched();
-}
-
-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();
+
+ /* 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)
@@ -163,6 +178,8 @@ 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);
@@ -262,12 +279,17 @@ static void xen_write_ldt_entry(struct d
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();
-}
-
-static int cvt_gate_to_trap(int vector, u32 low, u32 high, struct trap_info *info)
+
+ preempt_enable();
+}
+
+static int cvt_gate_to_trap(int vector, u32 low, u32 high,
+ struct trap_info *info)
{
u8 type, dpl;

@@ -295,11 +317,13 @@ static DEFINE_PER_CPU(struct Xgt_desc_st
also update Xen. */
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();

@@ -314,6 +338,8 @@ static void xen_write_idt_entry(struct d
if (HYPERVISOR_set_trap_table(info))
BUG();
}
+
+ preempt_enable();
}

static void xen_convert_trap_info(const struct Xgt_desc_struct *desc,
@@ -335,11 +361,9 @@ static void xen_convert_trap_info(const

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
@@ -349,11 +373,10 @@ static void xen_load_idt(const struct Xg
{
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);

@@ -368,6 +391,8 @@ static void xen_load_idt(const struct Xg
they're handled differently. */
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:
@@ -384,10 +409,12 @@ static void xen_write_gdt_entry(struct d
}

}
+
+ 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);
@@ -493,6 +520,8 @@ 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();
===================================================================
--- a/arch/i386/xen/mmu.c
+++ b/arch/i386/xen/mmu.c
@@ -481,5 +481,7 @@ void xen_exit_mmap(struct mm_struct *mm)

preempt_enable();

+ spin_lock(&mm->page_table_lock);
xen_pgd_unpin(mm->pgd);
-}
+ spin_unlock(&mm->page_table_lock);
+}
===================================================================
--- a/arch/i386/xen/multicalls.c
+++ b/arch/i386/xen/multicalls.c
@@ -1,4 +1,5 @@
#include <linux/percpu.h>
+#include <linux/hardirq.h>

#include <asm/xen/hypercall.h>

@@ -18,9 +19,11 @@ DEFINE_PER_CPU(unsigned long, xen_mc_irq

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 */
@@ -39,7 +42,6 @@ void xen_mc_flush(void)
} else
BUG_ON(b->argidx != 0);

- put_cpu_var(mc_buffer);
local_irq_restore(flags);

BUG_ON(ret);
@@ -47,10 +49,11 @@ 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 ||
@@ -62,7 +65,5 @@ struct multicall_space __xen_mc_entry(si
ret.args = &b->args[b->argidx];
b->argidx += argspace;

- put_cpu_var(mc_buffer);
-
return ret;
}
===================================================================
--- a/arch/i386/xen/time.c
+++ b/arch/i386/xen/time.c
@@ -48,7 +48,7 @@ static void get_runstate_snapshot(struct
u64 state_time;
struct vcpu_runstate_info *state;

- preempt_disable();
+ BUG_ON(preemptible());

state = &__get_cpu_var(runstate);

@@ -58,8 +58,6 @@ static void get_runstate_snapshot(struct
*res = *state;
barrier();
} while(state->state_entry_time != state_time);
-
- preempt_enable();
}

static void setup_runstate_info(void)
@@ -128,15 +126,29 @@ unsigned long long xen_sched_clock(void)
unsigned long long xen_sched_clock(void)
{
struct vcpu_runstate_info state;
- cycle_t now = xen_clocksource_read();
+ cycle_t now;
+ unsigned long long ret;
+
+ /*
+ * 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);

- return state.time[RUNSTATE_blocked] +
+ ret = state.time[RUNSTATE_blocked] +
state.time[RUNSTATE_running] +
(now - state.state_entry_time);
+
+ preempt_enable();
+
+ return ret;
}


--

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