[PATCH 2/5] KVM: Implement CR read caching for KVM paravirt_ops

Anthony Liguori
2007-06-18T02:58:57Z

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Regards,

Anthony Liguori

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Subject: [PATCH] KVM: Implement CR read caching for KVM paravirt_ops
Author: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

With hardware virtualization, CR reads often times require a VMEXIT which is
rather expensive. Instead of reading CR and taking the VMEXIT, maintain a
copy of each CR and return that on CR reads.

Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

diff --git a/arch/i386/kernel/kvm.c b/arch/i386/kernel/kvm.c
index 22ea647..89e83a4 100644
--- a/arch/i386/kernel/kvm.c
+++ b/arch/i386/kernel/kvm.c
@@ -26,8 +26,13 @@
#include <linux/cpu.h>
#include <linux/mm.h>

+#define CR0_TS_MASK (1ULL << 3)
+
struct kvm_paravirt_state
{
+ unsigned long cached_cr[5];
+ int cr_valid[5];
+
struct kvm_vmca *vmca;
struct kvm_hypercall_entry *queue;
void (*hypercall)(void);
@@ -37,6 +42,7 @@ struct kvm_paravirt_state

static DEFINE_PER_CPU(struct kvm_paravirt_state *, paravirt_state);

+static int do_cr_read_caching;
static int do_nop_io_delay;
static u64 msr_set_vmca;

@@ -69,6 +75,85 @@ static void kvm_io_delay(void)
{
}

+/*
+ * Control register reads can be trapped. Since trapping is relatively
+ * expensive, we can avoid paying the cost by caching logically.
+ */
+static unsigned long kvm_read_cr(int reg)
+{
+ struct kvm_paravirt_state *state
+ = per_cpu(paravirt_state, smp_processor_id());
+
+ if (unlikely(!state->cr_valid[reg])) {
+ if (reg == 0)
+ state->cached_cr[reg] = native_read_cr0();
+ else if (reg == 3)
+ state->cached_cr[reg] = native_read_cr3();
+ else if (reg == 4)
+ state->cached_cr[reg] = native_read_cr4();
+ else
+ BUG();
+ state->cr_valid[reg] = 1;
+ }
+ return state->cached_cr[reg];
+}
+
+static void kvm_write_cr(int reg, unsigned long value)
+{
+ struct kvm_paravirt_state *state
+ = per_cpu(paravirt_state, smp_processor_id());
+
+ state->cr_valid[reg] = 1;
+ state->cached_cr[reg] = value;
+
+ if (reg == 0)
+ native_write_cr0(value);
+ else if (reg == 3)
+ native_write_cr3(value);
+ else if (reg == 4)
+ native_write_cr4(value);
+ else
+ BUG();
+}
+
+static unsigned long kvm_read_cr0(void)
+{
+ return kvm_read_cr(0);
+}
+
+static void kvm_write_cr0(unsigned long value)
+{
+ kvm_write_cr(0, value);
+}
+
+/*
+ * We trap clts to ensure that our cached cr0 remains consistent.
+ */
+static void kvm_clts(void)
+{
+ write_cr0(read_cr0() & ~CR0_TS_MASK);
+}
+
+static unsigned long kvm_read_cr3(void)
+{
+ return kvm_read_cr(3);
+}
+
+static void kvm_write_cr3(unsigned long value)
+{
+ kvm_write_cr(3, value);
+}
+
+static unsigned long kvm_read_cr4(void)
+{
+ return kvm_read_cr(4);
+}
+
+static void kvm_write_cr4(unsigned long value)
+{
+ kvm_write_cr(4, value);
+}
+
static void paravirt_ops_setup(void)
{
paravirt_ops.name = "KVM";
@@ -76,6 +161,19 @@ static void paravirt_ops_setup(void)
if (do_nop_io_delay)
paravirt_ops.io_delay = kvm_io_delay;

+ if (do_cr_read_caching) {
+ paravirt_ops.clts = kvm_clts;
+ paravirt_ops.read_cr0 = kvm_read_cr0;
+ paravirt_ops.write_cr0 = kvm_write_cr0;
+ paravirt_ops.read_cr3 = kvm_read_cr3;
+ paravirt_ops.write_cr3 = kvm_write_cr3;
+ paravirt_ops.read_cr4 = kvm_read_cr4;
+ paravirt_ops.write_cr4 = kvm_write_cr4;
+
+ /* CR4 always exists in a KVM guest */
+ paravirt_ops.read_cr4_safe = kvm_read_cr4;
+ }
+
paravirt_ops.paravirt_enabled = 1;

apply_paravirt(__parainstructions, __parainstructions_end);
@@ -114,6 +212,9 @@ static int paravirt_initialize(void)
if ((edx & KVM_FEATURE_NOP_IO_DELAY))
do_nop_io_delay = 1;

+ if ((edx & KVM_FEATURE_CR_READ_CACHE))
+ do_cr_read_caching = 1;
+
on_each_cpu(paravirt_activate, NULL, 0, 1);

return 0;
diff --git a/drivers/kvm/kvm_main.c b/drivers/kvm/kvm_main.c
index f7a0e6e..7b57431 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -93,7 +93,8 @@ struct vfsmount *kvmfs_mnt;
#define EFER_RESERVED_BITS 0xfffffffffffff2fe

#define KVM_PARAVIRT_FEATURES \
- (KVM_FEATURE_VMCA | KVM_FEATURE_NOP_IO_DELAY)
+ (KVM_FEATURE_VMCA | KVM_FEATURE_NOP_IO_DELAY | \
+ KVM_FEATURE_CR_READ_CACHE)

#define KVM_MSR_SET_VMCA 0x87655678

diff --git a/include/linux/kvm_para.h b/include/linux/kvm_para.h
index cf51d4a..121a09c 100644
--- a/include/linux/kvm_para.h
+++ b/include/linux/kvm_para.h
@@ -13,6 +13,7 @@

#define KVM_FEATURE_VMCA (1UL << 0)
#define KVM_FEATURE_NOP_IO_DELAY (1UL << 1)
+#define KVM_FEATURE_CR_READ_CACHE (1UL << 2)

struct kvm_vmca
{

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