[PATCH] x86/hvm: accelerate I/O intercept handling

[Andre Przywara <andre.przywara@amd.com>]

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Hi,

currently we go through the emulator every time a HVM guest does an I/O 
port access (in/out). This is unnecessary most of the times, as both VMX 
and SVM provide all the necessary information already in the VMCS/VMCB. 
String instructions are not covered by this shortcut, but they are quite 
rare and we would need to access the guest memory anyway.
This patch decodes the information from VMCB/VMCS and calls a simple 
handle_mmio wrapper. In handle_mmio() itself the emulation part will 
simply be skipped, this approach avoids code duplication.
Since the vendor specific part is quite trivial, I implemented both the 
VMX and SVM part, please check the VMX part for sanity.

I boot-tested both versions and ran some simple benchmarks.
A micro benchmark (hammering an I/O port in a tight loop) shows a 
significant performance improvement (down to 66% of the time needed to 
handle the intercept on an AMD K8, measured in the guest with TSC).
Even with reading a 1GB file from an emulated IDE harddisk (Dom0 cached) 
I could get a  4-5% improvement.
Some guest code (e.g. the TCP stack in some Windows version) exercises 
the PM-Timer I/O port (0x1F48) very often (multiple 10,000 times per 
second), these workloads also benefit with up to 5% improvement from 
this patch.

Signed-off-by: Andre Przywara <andre.przywara@amd.com>

Regards,
Andre.

-- 
Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 448-3567-12

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diff -r ebd84be3420a xen/arch/x86/hvm/emulate.c
--- a/xen/arch/x86/hvm/emulate.c	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/arch/x86/hvm/emulate.c	Wed Mar 31 10:17:30 2010 +0200
@@ -48,7 +48,7 @@
     trace_var(event, 0/*!cycles*/, size, buffer);
 }
 
-static int hvmemul_do_io(
+int hvmemul_do_io(
     int is_mmio, paddr_t addr, unsigned long *reps, int size,
     paddr_t ram_gpa, int dir, int df, void *p_data)
 {
diff -r ebd84be3420a xen/arch/x86/hvm/io.c
--- a/xen/arch/x86/hvm/io.c	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/arch/x86/hvm/io.c	Wed Mar 31 10:17:30 2010 +0200
@@ -171,10 +171,22 @@
     struct hvm_emulate_ctxt ctxt;
     struct vcpu *curr = current;
     int rc;
+    unsigned long data, reps = 1;
 
-    hvm_emulate_prepare(&ctxt, guest_cpu_user_regs());
-
-    rc = hvm_emulate_one(&ctxt);
+    if ( curr->arch.hvm_vcpu.io_size == 0 ) {
+        hvm_emulate_prepare(&ctxt, guest_cpu_user_regs());
+        rc = hvm_emulate_one(&ctxt);
+    } else {
+        if ( curr->arch.hvm_vcpu.io_dir == 0 )
+            data = guest_cpu_user_regs()->eax;
+        rc = hvmemul_do_io(0, curr->arch.hvm_vcpu.io_port, &reps,
+                           curr->arch.hvm_vcpu.io_size, 0,
+                           curr->arch.hvm_vcpu.io_dir, 0, &data);
+        if ( curr->arch.hvm_vcpu.io_dir == 1 && rc == X86EMUL_OKAY ) {
+            memcpy(&(guest_cpu_user_regs()->eax),
+                   &data, curr->arch.hvm_vcpu.io_size);
+        }
+    }
 
     if ( curr->arch.hvm_vcpu.io_state == HVMIO_awaiting_completion )
         curr->arch.hvm_vcpu.io_state = HVMIO_handle_mmio_awaiting_completion;
@@ -184,14 +196,21 @@
     switch ( rc )
     {
     case X86EMUL_UNHANDLEABLE:
-        gdprintk(XENLOG_WARNING,
-                 "MMIO emulation failed @ %04x:%lx: "
-                 "%02x %02x %02x %02x %02x %02x\n",
-                 hvmemul_get_seg_reg(x86_seg_cs, &ctxt)->sel,
-                 ctxt.insn_buf_eip,
-                 ctxt.insn_buf[0], ctxt.insn_buf[1],
-                 ctxt.insn_buf[2], ctxt.insn_buf[3],
-                 ctxt.insn_buf[4], ctxt.insn_buf[5]);
+        if ( curr->arch.hvm_vcpu.io_size == 0 )
+            gdprintk(XENLOG_WARNING,
+                     "MMIO emulation failed @ %04x:%lx: "
+                     "%02x %02x %02x %02x %02x %02x\n",
+                     hvmemul_get_seg_reg(x86_seg_cs, &ctxt)->sel,
+                     ctxt.insn_buf_eip,
+                     ctxt.insn_buf[0], ctxt.insn_buf[1],
+                     ctxt.insn_buf[2], ctxt.insn_buf[3],
+                     ctxt.insn_buf[4], ctxt.insn_buf[5]);
+        else
+            gdprintk(XENLOG_WARNING,
+                     "I/O emulation failed: %s 0x%04x, %i bytes, data=%08lx\n",
+                      curr->arch.hvm_vcpu.io_dir ? "in" : "out",
+                      curr->arch.hvm_vcpu.io_port,
+                      curr->arch.hvm_vcpu.io_size, data);
         return 0;
     case X86EMUL_EXCEPTION:
         if ( ctxt.exn_pending )
@@ -201,9 +220,15 @@
         break;
     }
 
-    hvm_emulate_writeback(&ctxt);
+    if ( curr->arch.hvm_vcpu.io_size == 0 )
+        hvm_emulate_writeback(&ctxt);
+    else
+        curr->arch.hvm_vcpu.io_size = 0;
 
-    return 1;
+    if (rc == X86EMUL_RETRY)
+        return rc;
+    else
+        return 1;
 }
 
 int handle_mmio_with_translation(unsigned long gva, unsigned long gpfn)
@@ -213,6 +238,14 @@
     return handle_mmio();
 }
 
+int handle_mmio_decoded(uint16_t port, int size, int dir)
+{
+    current->arch.hvm_vcpu.io_port = port;
+    current->arch.hvm_vcpu.io_size = size;
+    current->arch.hvm_vcpu.io_dir = dir;
+    return handle_mmio();
+}
+
 void hvm_io_assist(void)
 {
     struct vcpu *curr = current;
diff -r ebd84be3420a xen/arch/x86/hvm/svm/emulate.c
--- a/xen/arch/x86/hvm/svm/emulate.c	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/arch/x86/hvm/svm/emulate.c	Wed Mar 31 10:17:30 2010 +0200
@@ -149,6 +149,9 @@
     if ( (inst_len = svm_nextrip_insn_length(v)) != 0 )
         return inst_len;
 
+    if ( vmcb->exitcode == VMEXIT_IOIO )
+        return vmcb->exitinfo2 - vmcb->rip;
+
     /* Fetch up to the next page break; we'll fetch from the next page
      * later if we have to. */
     fetch_addr = svm_rip2pointer(v);
diff -r ebd84be3420a xen/arch/x86/hvm/svm/svm.c
--- a/xen/arch/x86/hvm/svm/svm.c	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/arch/x86/hvm/svm/svm.c	Wed Mar 31 10:17:30 2010 +0200
@@ -42,6 +42,7 @@
 #include <asm/hvm/hvm.h>
 #include <asm/hvm/support.h>
 #include <asm/hvm/io.h>
+#include <asm/hvm/emulate.h>
 #include <asm/hvm/svm/asid.h>
 #include <asm/hvm/svm/svm.h>
 #include <asm/hvm/svm/vmcb.h>
@@ -1244,6 +1245,23 @@
     __update_guest_eip(regs, inst_len);
 }
 
+static int svm_vmexit_do_io(struct vmcb_struct *vmcb,
+                             struct cpu_user_regs *regs)
+{
+    uint16_t port;
+    int bytes, dir;
+    int rc;
+
+    port = (vmcb->exitinfo1 >> 16) & 0xFFFF;
+    bytes = ((vmcb->exitinfo1 >> 4) & 0x07);
+    dir = (vmcb->exitinfo1 & 1) ? IOREQ_READ : IOREQ_WRITE;
+
+    rc = handle_mmio_decoded(port, bytes, dir);
+    if ( rc != X86EMUL_RETRY )
+        __update_guest_eip(regs, vmcb->exitinfo2 - vmcb->rip);
+    return rc;
+}
+
 static void svm_invlpg_intercept(unsigned long vaddr)
 {
     struct vcpu *curr = current;
@@ -1450,11 +1468,17 @@
         svm_vmexit_do_hlt(vmcb, regs);
         break;
 
+    case VMEXIT_IOIO:
+        if ( ( vmcb->exitinfo1 & ( 1 << 2 ) ) == 0 ) {
+            if ( !svm_vmexit_do_io(vmcb, regs) )
+                hvm_inject_exception(TRAP_gp_fault, 0, 0);
+            break;
+        }
+        /* fallthrough to emulation if a string instruction */
     case VMEXIT_CR0_READ ... VMEXIT_CR15_READ:
     case VMEXIT_CR0_WRITE ... VMEXIT_CR15_WRITE:
     case VMEXIT_INVLPG:
     case VMEXIT_INVLPGA:
-    case VMEXIT_IOIO:
         if ( !handle_mmio() )
             hvm_inject_exception(TRAP_gp_fault, 0, 0);
         break;
diff -r ebd84be3420a xen/arch/x86/hvm/vmx/vmx.c
--- a/xen/arch/x86/hvm/vmx/vmx.c	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/arch/x86/hvm/vmx/vmx.c	Wed Mar 31 10:17:30 2010 +0200
@@ -1721,6 +1721,25 @@
     return 1;
 }
 
+static int vmx_io_intercept(unsigned long exit_qualification,
+                            struct cpu_user_regs *regs)
+{
+    uint16_t port;
+    int bytes, dir;
+    int rc;
+    int inst_len;
+
+    port = (exit_qualification >> 16) & 0xFFFF;
+    bytes = (exit_qualification & 0x07) + 1;
+    dir = (exit_qualification & 0x08) ? IOREQ_READ : IOREQ_WRITE;
+
+    inst_len = __get_instruction_length();
+    rc = handle_mmio_decoded(port, bytes, dir);
+    if ( rc != X86EMUL_RETRY)
+        __update_guest_eip(inst_len);
+    return rc;
+}
+
 static const struct lbr_info {
     u32 base, count;
 } p4_lbr[] = {
@@ -2589,8 +2608,14 @@
         break;
 
     case EXIT_REASON_IO_INSTRUCTION:
-        if ( !handle_mmio() )
-            vmx_inject_hw_exception(TRAP_gp_fault, 0);
+        exit_qualification = __vmread(EXIT_QUALIFICATION);
+        if (exit_qualification & 0x10) {
+            if ( !handle_mmio() )
+                vmx_inject_hw_exception(TRAP_gp_fault, 0);
+        } else {
+            if ( !vmx_io_intercept(exit_qualification, regs) )
+                vmx_inject_hw_exception(TRAP_gp_fault, 0);
+        }
         break;
 
     case EXIT_REASON_INVD:
diff -r ebd84be3420a xen/include/asm-x86/hvm/emulate.h
--- a/xen/include/asm-x86/hvm/emulate.h	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/include/asm-x86/hvm/emulate.h	Wed Mar 31 10:17:30 2010 +0200
@@ -46,4 +46,8 @@
     enum x86_segment seg,
     struct hvm_emulate_ctxt *hvmemul_ctxt);
 
+int hvmemul_do_io(
+    int is_mmio, paddr_t addr, unsigned long *reps, int size,
+    paddr_t ram_gpa, int dir, int df, void *p_data);
+
 #endif /* __ASM_X86_HVM_EMULATE_H__ */
diff -r ebd84be3420a xen/include/asm-x86/hvm/io.h
--- a/xen/include/asm-x86/hvm/io.h	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/include/asm-x86/hvm/io.h	Wed Mar 31 10:17:30 2010 +0200
@@ -100,6 +100,7 @@
 void send_invalidate_req(void);
 int handle_mmio(void);
 int handle_mmio_with_translation(unsigned long gva, unsigned long gpfn);
+int handle_mmio_decoded(uint16_t port, int size, int dir);
 void hvm_interrupt_post(struct vcpu *v, int vector, int type);
 void hvm_io_assist(void);
 void hvm_dpci_eoi(struct domain *d, unsigned int guest_irq,
diff -r ebd84be3420a xen/include/asm-x86/hvm/vcpu.h
--- a/xen/include/asm-x86/hvm/vcpu.h	Tue Mar 30 18:31:39 2010 +0100
+++ b/xen/include/asm-x86/hvm/vcpu.h	Wed Mar 31 10:17:30 2010 +0200
@@ -103,6 +103,10 @@
      */
     unsigned long       mmio_gva;
     unsigned long       mmio_gpfn;
+    uint16_t            io_port;
+    int                 io_size;
+    unsigned            io_dir;
+
     /* Callback into x86_emulate when emulating FPU/MMX/XMM instructions. */
     void (*fpu_exception_callback)(void *, struct cpu_user_regs *);
     void *fpu_exception_callback_arg;

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