From: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
During tracing the emulator, we noticed that init_emulate_ctxt()
sometimes took a bit longer time than we expected.
This patch is for mitigating the problem by some degree.
By looking into the function, we soon notice that it clears the whole
decode_cache whose size is about 2.5K bytes now. Furthermore, most of
the bytes are taken for the two read_cache arrays, which are used only
by a few instructions.
Considering the fact that we are not assuming the cache arrays have
been cleared when we store actual data, we do not need to clear the
arrays: 2K bytes elimination. In addition, we can avoid clearing the
fetch_cache and regs arrays.
This patch changes the initialization not to clear the arrays.
On our 64-bit host, init_emulate_ctxt() becomes 0.3 to 0.5us faster with
this patch applied.
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
Cc: Gleb Natapov <gleb@xxxxxxxxxx>
---
arch/x86/include/asm/kvm_emulate.h | 5 +++--
arch/x86/kvm/x86.c | 17 +++++++++++++++--
2 files changed, 18 insertions(+), 4 deletions(-)
diff --git a/arch/x86/include/asm/kvm_emulate.h b/arch/x86/include/asm/kvm_emulate.h
index ab09ba2..c0f77e0 100644
--- a/arch/x86/include/asm/kvm_emulate.h
+++ b/arch/x86/include/asm/kvm_emulate.h
@@ -246,8 +246,6 @@ struct decode_cache {
unsigned int d;
int (*execute)(struct x86_emulate_ctxt *ctxt);
int (*check_perm)(struct x86_emulate_ctxt *ctxt);
- unsigned long regs[NR_VCPU_REGS];
- unsigned long eip;
/* modrm */
u8 modrm;
u8 modrm_mod;
@@ -255,6 +253,9 @@ struct decode_cache {
u8 modrm_rm;
u8 modrm_seg;
bool rip_relative;
+ unsigned long eip;
+ /* Fields above regs are cleared together. */
+ unsigned long regs[NR_VCPU_REGS];
struct fetch_cache fetch;
struct read_cache io_read;
struct read_cache mem_read;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e7d337e..57f4160 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4506,6 +4506,20 @@ static void inject_emulated_exception(struct kvm_vcpu *vcpu)
kvm_queue_exception(vcpu, ctxt->exception.vector);
}
+static void init_decode_cache(struct decode_cache *c,
+ const unsigned long *regs)
+{
+ memset(c, 0, offsetof(struct decode_cache, regs));
+ memcpy(c->regs, regs, sizeof(c->regs));
+
+ c->fetch.start = 0;
+ c->fetch.end = 0;
+ c->io_read.pos = 0;
+ c->io_read.end = 0;
+ c->mem_read.pos = 0;
+ c->mem_read.end = 0;
+}
+
static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
{
struct x86_emulate_ctxt *ctxt = &vcpu->arch.emulate_ctxt;
@@ -4531,8 +4545,7 @@ static void init_emulate_ctxt(struct kvm_vcpu *vcpu)
X86EMUL_MODE_PROT16;
ctxt->guest_mode = is_guest_mode(vcpu);
- memset(c, 0, sizeof(struct decode_cache));
- memcpy(c->regs, vcpu->arch.regs, sizeof c->regs);
+ init_decode_cache(c, vcpu->arch.regs);
vcpu->arch.emulate_regs_need_sync_from_vcpu = false;
}
--
1.7.4.1
--
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