xen svm: fix PAUSE instruction intercept

PATCH In review Christoph Egger 2 participants 3 messages Aug 12, 2010

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Christoph Egger

author

2010-08-12 09:22 UTC #0 lore raw

PATCH 1/1 xen svm: fix PAUSE instruction intercept

Hi!

Attached patch moves PAUSE instruction intercept handling into svm_vmexit_do_pause() analogous to the other instruction intercepts.

In svm_vmexit_do_pause() update the instruction pointer. This fixes strange PAUSE intercepts on UP guests.

Signed-off-by Christoph Egger <[email protected]>

P.S.: This bug has been found with nested virtualization.

Signature · 6 lines

---to satisfy European Law for business letters:
Advanced Micro Devices GmbH
Einsteinring 24, 85609 Dornach b. Muenchen
Geschaeftsfuehrer: Alberto Bozzo, Andrew Bowd
Sitz: Dornach, Gemeinde Aschheim, Landkreis Muenchen
Registergericht Muenchen, HRB Nr. 43632

xen/arch/x86/hvm/svm/emulate.c +3 -1

diff -r f45026ec8db5 xen/arch/x86/hvm/svm/emulate.c
@@ -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,  2, 0xf3, 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)

xen/arch/x86/hvm/svm/svm.c +17 -6

diff -r f45026ec8db5 xen/arch/x86/hvm/svm/svm.c
@@ -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:

xen/arch/x86/hvm/svm/vmcb.c +1 -1

diff -r f45026ec8db5 xen/arch/x86/hvm/svm/vmcb.c
@@ -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;
     }

xen/include/asm-x86/hvm/svm/emulate.h +1 -0

diff -r f45026ec8db5 xen/include/asm-x86/hvm/svm/emulate.h
@@ -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 */
 };

xen/include/asm-x86/hvm/svm/svm.h +2 -0

diff -r f45026ec8db5 xen/include/asm-x86/hvm/svm/svm.h
@@ -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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Jan Beulich

2010-08-12 10:07 UTC #1 lore raw

Quoted 3 lines

Quoted 1 line
Quoted 1 line · Christoph Egger
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.

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().

Jan

Christoph Egger

author

2010-08-12 12:50 UTC #2 lore raw

Quoted 9 lines · On Thursday 12 August 2010 12:07:54 Jan Beulich wrote:

Quoted 4 lines
Quoted 1 line
Quoted 1 line · Christoph Egger
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.