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Subject[patch 32/33] xen: Place vcpu_info structure into per-cpu memory, if possible
An experimental patch for Xen allows guests to place their vcpu_info
structs anywhere. We try to use this to place the vcpu_info into the
PDA, which allows direct access.

If this works, then switch to using direct access operations for
irq_enable, disable, save_fl and restore_fl.

Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Chris Wright <chrisw@sous-sol.org>
Cc: Keir Fraser <keir@xensource.com>
---
arch/i386/xen/enlighten.c | 121 +++++++++++++++++++++++++++++++++++++++++-
arch/i386/xen/setup.c | 8 --
include/xen/interface/vcpu.h | 13 ++++
3 files changed, 133 insertions(+), 9 deletions(-)
===================================================================
--- a/arch/i386/xen/enlighten.c
+++ b/arch/i386/xen/enlighten.c
@@ -61,9 +61,55 @@ struct start_info *xen_start_info;
struct start_info *xen_start_info;
EXPORT_SYMBOL_GPL(xen_start_info);

+static /* __initdata */ struct shared_info dummy_shared_info;
+
+/*
+ * Point at some empty memory to start with. We map the real shared_info
+ * page as soon as fixmap is up and running.
+ */
+struct shared_info *HYPERVISOR_shared_info = (void *)&dummy_shared_info;
+
+/* tristate - -1: not tested, 0: not available, 1: available */
+static int have_vcpu_info_placement = -1;
+
void xen_vcpu_setup(int cpu)
{
+ struct vcpu_register_vcpu_info info;
+ int err;
+ struct vcpu_info *vcpup;
+
per_cpu(xen_vcpu, cpu) = &HYPERVISOR_shared_info->vcpu_info[cpu];
+
+ if (!have_vcpu_info_placement)
+ return; /* already tested, not available */
+
+ vcpup = &per_cpu(xen_vcpu_info, cpu);
+
+ printk("setting up vcpu for %d: xen_vcpu_info=%p\n",
+ cpu, vcpup);
+
+ info.mfn = virt_to_mfn(vcpup);
+ info.offset = offset_in_page(vcpup);
+
+ printk("trying to map vcpu_info %d at %p, mfn %x, offset %d\n",
+ cpu, vcpup, info.mfn, info.offset);
+
+ /* Check to see if the hypervisor will put the vcpu_info
+ structure where we want it, which allows direct access via
+ a percpu-variable. */
+ err = HYPERVISOR_vcpu_op(VCPUOP_register_vcpu_info, cpu, &info);
+
+ BUG_ON(err != 0 && err != -ENOSYS);
+ BUG_ON(err && have_vcpu_info_placement==1); /* all or nothing */
+
+ have_vcpu_info_placement = 0;
+
+ if (err == 0) {
+ have_vcpu_info_placement = 1;
+ per_cpu(xen_vcpu, cpu) = vcpup;
+ printk("cpu %d using vcpu_info at %p\n",
+ cpu, vcpup);
+ }
}

static void __init xen_banner(void)
@@ -123,6 +169,20 @@ static unsigned long xen_save_fl(void)
return (-flags) & X86_EFLAGS_IF;
}

+static unsigned long xen_save_fl_direct(void)
+{
+ unsigned long flags;
+
+ /* flag has opposite sense of mask */
+ flags = !x86_read_percpu(xen_vcpu_info.evtchn_upcall_mask);
+
+ /* convert to IF type flag
+ -0 -> 0x00000000
+ -1 -> 0xffffffff
+ */
+ return (-flags) & X86_EFLAGS_IF;
+}
+
static void xen_restore_fl(unsigned long flags)
{
struct vcpu_info *vcpu;
@@ -149,6 +209,25 @@ static void xen_restore_fl(unsigned long
}
}

+static void xen_restore_fl_direct(unsigned long flags)
+{
+ /* convert from IF type flag */
+ flags = !(flags & X86_EFLAGS_IF);
+
+ /* This is an atomic update, so no need to worry about
+ preemption. */
+ x86_write_percpu(xen_vcpu_info.evtchn_upcall_mask, flags);
+
+ /* If we get preempted here, then any pending event will be
+ handled anyway. */
+
+ if (flags == 0) {
+ barrier(); /* unmask then check (avoid races) */
+ if (unlikely(x86_read_percpu(xen_vcpu_info.evtchn_upcall_pending)))
+ force_evtchn_callback();
+ }
+}
+
static void xen_irq_disable(void)
{
/* There's a one instruction preempt window here. We need to
@@ -157,6 +236,12 @@ static void xen_irq_disable(void)
preempt_disable();
x86_read_percpu(xen_vcpu)->evtchn_upcall_mask = 1;
preempt_enable_no_resched();
+}
+
+static void xen_irq_disable_direct(void)
+{
+ /* Atomic update, so preemption not a concern. */
+ x86_write_percpu(xen_vcpu_info.evtchn_upcall_mask, 1);
}

static void xen_irq_enable(void)
@@ -176,6 +261,19 @@ static void xen_irq_enable(void)

barrier(); /* unmask then check (avoid races) */
if (unlikely(vcpu->evtchn_upcall_pending))
+ force_evtchn_callback();
+}
+
+static void xen_irq_enable_direct(void)
+{
+ /* Atomic update, so preemption not a concern. */
+ x86_write_percpu(xen_vcpu_info.evtchn_upcall_mask, 0);
+
+ /* 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(x86_read_percpu(xen_vcpu_info.evtchn_upcall_pending)))
force_evtchn_callback();
}

@@ -538,9 +636,19 @@ static void xen_flush_tlb_others(const c
xen_mc_issue(PARAVIRT_LAZY_MMU);
}

+static void xen_write_cr2(unsigned long cr2)
+{
+ x86_read_percpu(xen_vcpu)->arch.cr2 = cr2;
+}
+
static unsigned long xen_read_cr2(void)
{
return x86_read_percpu(xen_vcpu)->arch.cr2;
+}
+
+static unsigned long xen_read_cr2_direct(void)
+{
+ return x86_read_percpu(xen_vcpu_info.arch.cr2);
}

static void xen_write_cr4(unsigned long cr4)
@@ -775,7 +883,7 @@ static const struct paravirt_ops xen_par
.write_cr0 = native_write_cr0,

.read_cr2 = xen_read_cr2,
- .write_cr2 = native_write_cr2,
+ .write_cr2 = xen_write_cr2,

.read_cr3 = xen_read_cr3,
.write_cr3 = xen_write_cr3,
@@ -963,6 +1071,17 @@ static asmlinkage void __init xen_start_
x86_write_percpu(xen_cr3, __pa(pgd));
xen_vcpu_setup(0);

+ /* xen_vcpu_setup managed to place the vcpu_info within the
+ pda, so make use of it */
+ BUG_ON(have_vcpu_info_placement == -1);
+ if (have_vcpu_info_placement) {
+ paravirt_ops.save_fl = xen_save_fl_direct;
+ paravirt_ops.restore_fl = xen_restore_fl_direct;
+ paravirt_ops.irq_disable = xen_irq_disable_direct;
+ paravirt_ops.irq_enable = xen_irq_enable_direct;
+ paravirt_ops.read_cr2 = xen_read_cr2_direct;
+ }
+
paravirt_ops.kernel_rpl = 1;
if (xen_feature(XENFEAT_supervisor_mode_kernel))
paravirt_ops.kernel_rpl = 0;
===================================================================
--- a/arch/i386/xen/setup.c
+++ b/arch/i386/xen/setup.c
@@ -22,14 +22,6 @@
/* These are code, but not functions. Defined in entry.S */
extern const char xen_hypervisor_callback[];
extern const char xen_failsafe_callback[];
-
-static __initdata struct shared_info init_shared;
-
-/*
- * Point at some empty memory to start with. We map the real shared_info
- * page as soon as fixmap is up and running.
- */
-struct shared_info *HYPERVISOR_shared_info = &init_shared;

unsigned long *phys_to_machine_mapping;
EXPORT_SYMBOL(phys_to_machine_mapping);
===================================================================
--- a/include/xen/interface/vcpu.h
+++ b/include/xen/interface/vcpu.h
@@ -151,6 +151,19 @@ struct vcpu_set_singleshot_timer {
#define _VCPU_SSHOTTMR_future (0)
#define VCPU_SSHOTTMR_future (1U << _VCPU_SSHOTTMR_future)

+/*
+ * Register a memory location in the guest address space for the
+ * vcpu_info structure. This allows the guest to place the vcpu_info
+ * structure in a convenient place, such as in a per-cpu data area.
+ * The pointer need not be page aligned, but the structure must not
+ * cross a page boundary.
+ */
+#define VCPUOP_register_vcpu_info 10 /* arg == struct vcpu_info */
+struct vcpu_register_vcpu_info {
+ uint32_t mfn; /* mfn of page to place vcpu_info */
+ uint32_t offset; /* offset within page */
+};
+
#endif /* __XEN_PUBLIC_VCPU_H__ */

/*
--

-
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