Regards,
Anthony Liguori
Subject: [PATCH] KVM: Implement CR read caching for KVM paravirt_ops
Author: Anthony Liguori <aliguori@xxxxxxxxxx>
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@xxxxxxxxxx>
diff --git a/arch/i386/kernel/kvm.c b/arch/i386/kernel/kvm.c
index 62c3b5b..dcc45cd 100644
--- a/arch/i386/kernel/kvm.c
+++ b/arch/i386/kernel/kvm.c
@@ -38,8 +38,13 @@
#define KVM_SCALE 22
+#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);
@@ -49,6 +54,7 @@ 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 int do_paravirt_clock;
static u64 msr_set_vmca;
@@ -104,6 +110,93 @@ 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 __always_inline unsigned long kvm_read_cr(int reg)
+{
+ struct kvm_paravirt_state *state = x86_read_percpu(paravirt_state);
+
+ if (unlikely(!state->cr_valid[reg])) {
+ switch (reg) {
+ case 0:
+ state->cached_cr[reg] = native_read_cr0();
+ break;
+ case 3:
+ state->cached_cr[reg] = native_read_cr3();
+ break;
+ case 4:
+ state->cached_cr[reg] = native_read_cr4();
+ break;
+ default:
+ BUG();
+ }
+ state->cr_valid[reg] = 1;
+ }
+ return state->cached_cr[reg];
+}
+
+static __always_inline void kvm_write_cr(int reg, unsigned long value)
+{
+ struct kvm_paravirt_state *state = x86_read_percpu(paravirt_state);
+
+ state->cr_valid[reg] = 1;
+ state->cached_cr[reg] = value;
+
+ switch (reg) {
+ case 0:
+ native_write_cr0(value);
+ break;
+ case 3:
+ native_write_cr3(value);
+ break;
+ case 4:
+ native_write_cr4(value);
+ break;
+ default:
+ 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";
@@ -119,6 +212,19 @@ static void paravirt_ops_setup(void)
printk(KERN_INFO "KVM: using paravirt clock source\n");
}
+ 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);
@@ -160,6 +266,9 @@ static int paravirt_initialize(void)
if ((edx & KVM_FEATURE_PARAVIRT_CLOCK))
do_paravirt_clock = 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 e455fca..35d73b8 100644
--- a/drivers/kvm/kvm_main.c
+++ b/drivers/kvm/kvm_main.c
@@ -95,7 +95,7 @@ struct vfsmount *kvmfs_mnt;
#define KVM_PARAVIRT_FEATURES \
(KVM_FEATURE_VMCA | KVM_FEATURE_NOP_IO_DELAY | \
- KVM_FEATURE_PARAVIRT_CLOCK)
+ KVM_FEATURE_PARAVIRT_CLOCK | 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 4f27fb1..be185da 100644
--- a/include/linux/kvm_para.h
+++ b/include/linux/kvm_para.h
@@ -16,6 +16,7 @@
#define KVM_FEATURE_VMCA (1UL << 0)
#define KVM_FEATURE_NOP_IO_DELAY (1UL << 1)
#define KVM_FEATURE_PARAVIRT_CLOCK (1UL << 2)
+#define KVM_FEATURE_CR_READ_CACHE (1UL << 3)
struct kvm_vmca
{
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