Re: [PATCH] xen svm: fix PAUSE instruction intercept

[Christoph Egger <Christoph.Egger@amd.com>]

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On Thursday 12 August 2010 12:07:54 Jan Beulich wrote:
> >>> On 12.08.10 at 11:22, Christoph Egger <Christoph.Egger@amd.com> wrote:
> >
> > In svm_vmexit_do_pause() update the instruction pointer.
> > This fixes strange PAUSE intercepts on UP guests.
>
> This won't work when the instruction needs to actually be decoded:
> __get_instruction_length_from_list() strips all prefix bytes from
> the byte stream being decoded, and since 'pause' uses a prefix the
> actual opcode comparison will never produce a match.

Thank you for pointing this out. New attached version fixes this.

Signed-off-by: Christoph Egger <Christoph.Egger@amd.com>

> Even in the case that CPUs supporting the pause filter would always
> support nextrip I think that the special case minimally deserves a
> code comment to that effect, but perhaps even warrants an
> ASSERT() or BUG_ON().

This is true for real hardware but not necessarily when it comes
to nested virtualization, because you emulate SVM features
in software.

Christoph


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diff -r f45026ec8db5 xen/arch/x86/hvm/svm/emulate.c
--- a/xen/arch/x86/hvm/svm/emulate.c	Mon Aug 09 18:29:50 2010 +0100
+++ b/xen/arch/x86/hvm/svm/emulate.c	Thu Aug 12 14:45:24 2010 +0200
@@ -100,6 +100,7 @@ MAKE_INSTR(VMCALL, 3, 0x0f, 0x01, 0xd9);
 MAKE_INSTR(HLT,    1, 0xf4);
 MAKE_INSTR(INT3,   1, 0xcc);
 MAKE_INSTR(RDTSC,  2, 0x0f, 0x31);
+MAKE_INSTR(PAUSE,  1, 0x90);
 
 static const u8 *opc_bytes[INSTR_MAX_COUNT] = 
 {
@@ -111,7 +112,8 @@ static const u8 *opc_bytes[INSTR_MAX_COU
     [INSTR_VMCALL] = OPCODE_VMCALL,
     [INSTR_HLT]    = OPCODE_HLT,
     [INSTR_INT3]   = OPCODE_INT3,
-    [INSTR_RDTSC]  = OPCODE_RDTSC
+    [INSTR_RDTSC]  = OPCODE_RDTSC,
+    [INSTR_PAUSE]  = OPCODE_PAUSE,
 };
 
 static int fetch(struct vcpu *v, u8 *buf, unsigned long addr, int len)
diff -r f45026ec8db5 xen/arch/x86/hvm/svm/svm.c
--- a/xen/arch/x86/hvm/svm/svm.c	Mon Aug 09 18:29:50 2010 +0100
+++ b/xen/arch/x86/hvm/svm/svm.c	Thu Aug 12 14:45:24 2010 +0200
@@ -1261,6 +1261,22 @@ static void svm_vmexit_do_rdtsc(struct c
     hvm_rdtsc_intercept(regs);
 }
 
+static void svm_vmexit_do_pause(struct cpu_user_regs *regs)
+{
+    unsigned int inst_len;
+
+    if ( (inst_len = __get_instruction_length(current, INSTR_PAUSE)) == 0 )
+        return;
+    __update_guest_eip(regs, inst_len);
+
+    /*
+     * The guest is running a contended spinlock and we've detected it.
+     * Do something useful, like reschedule the guest
+     */
+    perfc_incr(pauseloop_exits);
+    do_sched_op_compat(SCHEDOP_yield, 0);
+}
+
 static void svm_vmexit_ud_intercept(struct cpu_user_regs *regs)
 {
     struct hvm_emulate_ctxt ctxt;
@@ -1655,12 +1671,7 @@ asmlinkage void svm_vmexit_handler(struc
         break;
 
     case VMEXIT_PAUSE:
-        /*
-         * The guest is running a contended spinlock and we've detected it.
-         * Do something useful, like reschedule the guest
-         */
-        perfc_incr(pauseloop_exits);
-        do_sched_op_compat(SCHEDOP_yield, 0);
+        svm_vmexit_do_pause(regs);
         break;
 
     default:
diff -r f45026ec8db5 xen/arch/x86/hvm/svm/vmcb.c
--- a/xen/arch/x86/hvm/svm/vmcb.c	Mon Aug 09 18:29:50 2010 +0100
+++ b/xen/arch/x86/hvm/svm/vmcb.c	Thu Aug 12 14:45:24 2010 +0200
@@ -253,7 +253,7 @@ static int construct_vmcb(struct vcpu *v
 
     if ( cpu_has_pause_filter )
     {
-        vmcb->pause_filter_count = 3000;
+        vmcb->pause_filter_count = SVM_PAUSEFILTER_INIT;
         vmcb->general1_intercepts |= GENERAL1_INTERCEPT_PAUSE;
     }
 
diff -r f45026ec8db5 xen/include/asm-x86/hvm/svm/emulate.h
--- a/xen/include/asm-x86/hvm/svm/emulate.h	Mon Aug 09 18:29:50 2010 +0100
+++ b/xen/include/asm-x86/hvm/svm/emulate.h	Thu Aug 12 14:45:24 2010 +0200
@@ -31,6 +31,7 @@ enum instruction_index {
     INSTR_HLT,
     INSTR_INT3,
     INSTR_RDTSC,
+    INSTR_PAUSE,
     INSTR_MAX_COUNT /* Must be last - Number of instructions supported */
 };
 
diff -r f45026ec8db5 xen/include/asm-x86/hvm/svm/svm.h
--- a/xen/include/asm-x86/hvm/svm/svm.h	Mon Aug 09 18:29:50 2010 +0100
+++ b/xen/include/asm-x86/hvm/svm/svm.h	Thu Aug 12 14:45:24 2010 +0200
@@ -76,4 +76,6 @@ extern u32 svm_feature_flags;
 #define cpu_has_svm_nrips   test_bit(SVM_FEATURE_NRIPS, &svm_feature_flags)
 #define cpu_has_pause_filter  test_bit(SVM_FEATURE_PAUSEF, &svm_feature_flags)
 
+#define SVM_PAUSEFILTER_INIT    3000
+
 #endif /* __ASM_X86_HVM_SVM_H__ */

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