[PATCH v5] x86/Intel: virtualize support for cpuid faulting

[PATCH v5] x86/Intel: virtualize support for cpuid faulting

[Kyle Huey]

rr (http://rr-project.org/), a Linux userspace record-and-replay reverse-
execution debugger, would like to trap and emulate the CPUID instruction.
This would allow us to a) mask away certain hardware features that rr does
not support (e.g. RDRAND) and b) enable trace portability across machines
by providing constant results. Patches for support in the Linux kernel are in
flight, and we'd like to be able to use this feature on virtualized Linux
instances as well.

Changes since v4:
- Renamed cpuid_fault to cpuid_faulting across the patch.
- Various style nits.
- Added Reviewed-by Kevin Tian.


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[PATCH v5 1/2] x86/Intel: Expose cpuid_faulting_enabled so it can be used elsewhere

[Kyle Huey]

While we're here, use bool instead of bool_t.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
---
 xen/arch/x86/cpu/intel.c    | 9 +++++----
 xen/include/asm-x86/cpuid.h | 3 +++
 2 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/cpu/intel.c b/xen/arch/x86/cpu/intel.c
index 7b60aaa..2e11662 100644
--- a/xen/arch/x86/cpu/intel.c
+++ b/xen/arch/x86/cpu/intel.c
@@ -13,34 +13,35 @@
 #include <mach_apic.h>
 #include <asm/hvm/support.h>
 #include <asm/setup.h>
 
 #include "cpu.h"
 
 #define select_idle_routine(x) ((void)0)
 
-static bool_t __init probe_intel_cpuid_faulting(void)
+static bool __init probe_intel_cpuid_faulting(void)
 {
 	uint64_t x;
 
 	if (rdmsr_safe(MSR_INTEL_PLATFORM_INFO, x) ||
 	    !(x & MSR_PLATFORM_INFO_CPUID_FAULTING))
 		return 0;
 
 	expected_levelling_cap |= LCAP_faulting;
 	levelling_caps |=  LCAP_faulting;
 	__set_bit(X86_FEATURE_CPUID_FAULTING, boot_cpu_data.x86_capability);
 	return 1;
 }
 
-static void set_cpuid_faulting(bool_t enable)
+DEFINE_PER_CPU(bool, cpuid_faulting_enabled);
+
+static void set_cpuid_faulting(bool enable)
 {
-	static DEFINE_PER_CPU(bool_t, cpuid_faulting_enabled);
-	bool_t *this_enabled = &this_cpu(cpuid_faulting_enabled);
+	bool *this_enabled = &this_cpu(cpuid_faulting_enabled);
 	uint32_t hi, lo;
 
 	ASSERT(cpu_has_cpuid_faulting);
 
 	if (*this_enabled == enable)
 		return;
 
 	rdmsr(MSR_INTEL_MISC_FEATURES_ENABLES, lo, hi);
diff --git a/xen/include/asm-x86/cpuid.h b/xen/include/asm-x86/cpuid.h
index 8e3f639..2372474 100644
--- a/xen/include/asm-x86/cpuid.h
+++ b/xen/include/asm-x86/cpuid.h
@@ -59,16 +59,19 @@ struct cpuidmasks
 };
 
 /* Per CPU shadows of masking MSR values, for lazy context switching. */
 DECLARE_PER_CPU(struct cpuidmasks, cpuidmasks);
 
 /* Default masking MSR values, calculated at boot. */
 extern struct cpuidmasks cpuidmask_defaults;
 
+/* Whether or not cpuid faulting is available for the current domain. */
+DECLARE_PER_CPU(bool, cpuid_faulting_enabled);
+
 #endif /* __ASSEMBLY__ */
 #endif /* !__X86_CPUID_H__ */
 
 /*
  * Local variables:
  * mode: C
  * c-file-style: "BSD"
  * c-basic-offset: 4

base-commit: 20295ab63ce7f57edca9c450602ac2dace1fc721
prerequisite-patch-id: 542330a7d654761b9b641741f31d1f8bcb92f3e1
prerequisite-patch-id: 01816b228ec9df800b3f89fbef850e53e8692a9d
prerequisite-patch-id: efc4d299fa89b26ef20dcea7e3f9ca57a08fc01a
prerequisite-patch-id: cd423fb9cf7c900491b8d280da6e7cc4dbd2f53f
prerequisite-patch-id: 47d975de5d9d2f362e1cb283f7342a79d05265ca
-- 
2.10.1


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[PATCH v5 2/2] x86/Intel: virtualize support for cpuid faulting

[Kyle Huey]

On HVM guests, the cpuid triggers a vm exit, so we can check the emulated
faulting state in vmx_do_cpuid and hvmemul_cpuid. A new function,
hvm_check_cpuid_fault will check if cpuid faulting is enabled and the CPL > 0.
When it returns true, the cpuid handling functions will inject a GP(0). Notably
explicit hardware support for faulting on cpuid is not necessary to emulate
support for an HVM guest.

On PV guests, hardware support is required so that userspace cpuid will trap
to Xen. Xen already enables cpuid faulting on supported CPUs for pv guests (that
aren't the control domain, see the comment in intel_ctxt_switch_levelling).
Every PV guest cpuid will trap via a GP(0) to emulate_privileged_op (via
do_general_protection). Once there we simply decline to emulate cpuid if the
CPL > 0 and faulting is enabled, leaving the GP(0) for the guest kernel to
handle.

Signed-off-by: Kyle Huey <khuey@kylehuey.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
---
 xen/arch/x86/hvm/emulate.c    | 20 ++++++++++++++++++++
 xen/arch/x86/hvm/hvm.c        | 14 ++++++++++++++
 xen/arch/x86/hvm/vmx/vmx.c    | 21 +++++++++++++++++++--
 xen/arch/x86/traps.c          | 35 +++++++++++++++++++++++++++++++++++
 xen/include/asm-x86/domain.h  |  3 +++
 xen/include/asm-x86/hvm/hvm.h |  1 +
 6 files changed, 92 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/emulate.c b/xen/arch/x86/hvm/emulate.c
index 6ed7486..70c8d44 100644
--- a/xen/arch/x86/hvm/emulate.c
+++ b/xen/arch/x86/hvm/emulate.c
@@ -1544,16 +1544,36 @@ static int hvmemul_wbinvd(
 
 static int hvmemul_cpuid(
     unsigned int *eax,
     unsigned int *ebx,
     unsigned int *ecx,
     unsigned int *edx,
     struct x86_emulate_ctxt *ctxt)
 {
+    /*
+     * x86_emulate uses this function to query CPU features for its own internal
+     * use. Make sure we're actually emulating CPUID before emulating CPUID
+     * faulting.
+     */
+    if ( ctxt->opcode == X86EMUL_OPC(0x0f, 0xa2) &&
+         hvm_check_cpuid_faulting(current) )
+    {
+        struct hvm_emulate_ctxt *hvmemul_ctxt =
+            container_of(ctxt, struct hvm_emulate_ctxt, ctxt);
+
+        hvmemul_ctxt->exn_pending = 1;
+        hvmemul_ctxt->trap.vector = TRAP_gp_fault;
+        hvmemul_ctxt->trap.type = X86_EVENTTYPE_HW_EXCEPTION;
+        hvmemul_ctxt->trap.error_code = 0;
+        hvmemul_ctxt->trap.insn_len = 0;
+
+        return X86EMUL_EXCEPTION;
+    }
+
     hvm_funcs.cpuid_intercept(eax, ebx, ecx, edx);
     return X86EMUL_OKAY;
 }
 
 static int hvmemul_inject_hw_exception(
     uint8_t vector,
     int32_t error_code,
     struct x86_emulate_ctxt *ctxt)
diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
index 3c90ecd..11e2b82 100644
--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -3675,16 +3675,30 @@ void hvm_cpuid(unsigned int input, unsigned int *eax, unsigned int *ebx,
         hvm_cpuid(0x80000001, NULL, NULL, NULL, &_edx);
         *eax |= (_edx & cpufeat_mask(X86_FEATURE_LM) ? vaddr_bits : 32) << 8;
 
         *ebx &= hvm_featureset[FEATURESET_e8b];
         break;
     }
 }
 
+bool hvm_check_cpuid_faulting(struct vcpu *v)
+{
+    struct segment_register sreg;
+
+    if ( !v->arch.cpuid_faulting )
+        return false;
+
+    hvm_get_segment_register(v, x86_seg_ss, &sreg);
+    if ( sreg.attr.fields.dpl == 0 )
+        return false;
+
+    return true;
+}
+
 static uint64_t _hvm_rdtsc_intercept(void)
 {
     struct vcpu *curr = current;
 #if !defined(NDEBUG) || defined(CONFIG_PERF_COUNTERS)
     struct domain *currd = curr->domain;
 
     if ( currd->arch.vtsc )
         switch ( hvm_guest_x86_mode(curr) )
diff --git a/xen/arch/x86/hvm/vmx/vmx.c b/xen/arch/x86/hvm/vmx/vmx.c
index b9102ce..c3176a1 100644
--- a/xen/arch/x86/hvm/vmx/vmx.c
+++ b/xen/arch/x86/hvm/vmx/vmx.c
@@ -2428,16 +2428,22 @@ static void vmx_cpuid_intercept(
     HVMTRACE_5D (CPUID, input, *eax, *ebx, *ecx, *edx);
 }
 
 static int vmx_do_cpuid(struct cpu_user_regs *regs)
 {
     unsigned int eax, ebx, ecx, edx;
     unsigned int leaf, subleaf;
 
+    if ( hvm_check_cpuid_faulting(current) )
+    {
+        hvm_inject_hw_exception(TRAP_gp_fault, 0);
+        return 1;  /* Don't advance the guest IP! */
+    }
+
     eax = regs->eax;
     ebx = regs->ebx;
     ecx = regs->ecx;
     edx = regs->edx;
 
     leaf = regs->eax;
     subleaf = regs->ecx;
 
@@ -2694,19 +2700,23 @@ static int vmx_msr_read_intercept(unsigned int msr, uint64_t *msr_content)
     case MSR_CORE_PERF_FIXED_CTR_CTRL...MSR_CORE_PERF_GLOBAL_OVF_CTRL:
     case MSR_IA32_PEBS_ENABLE:
     case MSR_IA32_DS_AREA:
         if ( vpmu_do_rdmsr(msr, msr_content) )
             goto gp_fault;
         break;
 
     case MSR_INTEL_PLATFORM_INFO:
-        if ( rdmsr_safe(MSR_INTEL_PLATFORM_INFO, *msr_content) )
-            goto gp_fault;
+        *msr_content = MSR_PLATFORM_INFO_CPUID_FAULTING;
+        break;
+
+    case MSR_INTEL_MISC_FEATURES_ENABLES:
         *msr_content = 0;
+        if ( current->arch.cpuid_faulting )
+            *msr_content |= MSR_MISC_FEATURES_CPUID_FAULTING;
         break;
 
     default:
         if ( passive_domain_do_rdmsr(msr, msr_content) )
             goto done;
         switch ( long_mode_do_msr_read(msr, msr_content) )
         {
             case HNDL_unhandled:
@@ -2925,16 +2935,23 @@ static int vmx_msr_write_intercept(unsigned int msr, uint64_t msr_content)
         break;
 
     case MSR_INTEL_PLATFORM_INFO:
         if ( msr_content ||
              rdmsr_safe(MSR_INTEL_PLATFORM_INFO, msr_content) )
             goto gp_fault;
         break;
 
+    case MSR_INTEL_MISC_FEATURES_ENABLES:
+        if ( msr_content & ~MSR_MISC_FEATURES_CPUID_FAULTING )
+            goto gp_fault;
+        v->arch.cpuid_faulting =
+            !!(msr_content & MSR_MISC_FEATURES_CPUID_FAULTING);
+        break;
+
     default:
         if ( passive_domain_do_wrmsr(msr, msr_content) )
             return X86EMUL_OKAY;
 
         if ( wrmsr_viridian_regs(msr, msr_content) ) 
             break;
 
         switch ( long_mode_do_msr_write(msr, msr_content) )
diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 293ff8d..63765ec 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -1315,16 +1315,25 @@ static int emulate_forced_invalid_op(struct cpu_user_regs *regs)
     /* We only emulate CPUID. */
     if ( ( rc = copy_from_user(instr, (char *)eip, sizeof(instr))) != 0 )
     {
         propagate_page_fault(eip + sizeof(instr) - rc, 0);
         return EXCRET_fault_fixed;
     }
     if ( memcmp(instr, "\xf\xa2", sizeof(instr)) )
         return 0;
+
+    /* If cpuid faulting is enabled and CPL>0 inject a #GP in place of #UD. */
+    if ( current->arch.cpuid_faulting && !guest_kernel_mode(current, regs) )
+    {
+        regs->eip = eip;
+        do_guest_trap(TRAP_gp_fault, regs);
+        return EXCRET_fault_fixed;
+    }
+
     eip += sizeof(instr);
 
     pv_cpuid(regs);
 
     instruction_done(regs, eip, 0);
 
     trace_trap_one_addr(TRC_PV_FORCED_INVALID_OP, regs->eip);
 
@@ -2474,16 +2483,27 @@ static int priv_op_read_msr(unsigned int reg, uint64_t *val,
         *val = 0;
         return X86EMUL_OKAY;
 
     case MSR_INTEL_PLATFORM_INFO:
         if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
              rdmsr_safe(MSR_INTEL_PLATFORM_INFO, *val) )
             break;
         *val = 0;
+        if ( this_cpu(cpuid_faulting_enabled) )
+            *val |= MSR_PLATFORM_INFO_CPUID_FAULTING;
+        return X86EMUL_OKAY;
+
+    case MSR_INTEL_MISC_FEATURES_ENABLES:
+        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+             rdmsr_safe(MSR_INTEL_MISC_FEATURES_ENABLES, *val) )
+            break;
+        *val = 0;
+        if ( curr->arch.cpuid_faulting )
+            *val |= MSR_MISC_FEATURES_CPUID_FAULTING;
         return X86EMUL_OKAY;
 
     case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
     case MSR_P6_EVNTSEL(0)...MSR_P6_EVNTSEL(3):
     case MSR_CORE_PERF_FIXED_CTR0...MSR_CORE_PERF_FIXED_CTR2:
     case MSR_CORE_PERF_FIXED_CTR_CTRL...MSR_CORE_PERF_GLOBAL_OVF_CTRL:
         if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL )
         {
@@ -2677,16 +2697,27 @@ static int priv_op_write_msr(unsigned int reg, uint64_t val,
         return X86EMUL_OKAY;
 
     case MSR_INTEL_PLATFORM_INFO:
         if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
              val || rdmsr_safe(MSR_INTEL_PLATFORM_INFO, val) )
             break;
         return X86EMUL_OKAY;
 
+    case MSR_INTEL_MISC_FEATURES_ENABLES:
+        if ( boot_cpu_data.x86_vendor != X86_VENDOR_INTEL ||
+             (val & ~MSR_MISC_FEATURES_CPUID_FAULTING) ||
+             rdmsr_safe(MSR_INTEL_MISC_FEATURES_ENABLES, temp) )
+            break;
+        if ( (val & MSR_MISC_FEATURES_CPUID_FAULTING) &&
+             !this_cpu(cpuid_faulting_enabled) )
+            break;
+        curr->arch.cpuid_faulting = !!(val & MSR_MISC_FEATURES_CPUID_FAULTING);
+        return X86EMUL_OKAY;
+
     case MSR_P6_PERFCTR(0)...MSR_P6_PERFCTR(7):
     case MSR_P6_EVNTSEL(0)...MSR_P6_EVNTSEL(3):
     case MSR_CORE_PERF_FIXED_CTR0...MSR_CORE_PERF_FIXED_CTR2:
     case MSR_CORE_PERF_FIXED_CTR_CTRL...MSR_CORE_PERF_GLOBAL_OVF_CTRL:
         if ( boot_cpu_data.x86_vendor == X86_VENDOR_INTEL )
         {
             vpmu_msr = true;
     case MSR_AMD_FAM15H_EVNTSEL0...MSR_AMD_FAM15H_PERFCTR5:
@@ -3186,16 +3217,20 @@ static int emulate_privileged_op(struct cpu_user_regs *regs)
         if ( priv_op_read_msr(regs->_ecx, &val, NULL) != X86EMUL_OKAY )
             goto fail;
  rdmsr_writeback:
         regs->eax = (uint32_t)val;
         regs->edx = (uint32_t)(val >> 32);
         break;
 
     case 0xa2: /* CPUID */
+        /* If cpuid faulting is enabled and CPL>0 leave the #GP untouched. */
+        if ( v->arch.cpuid_faulting && !guest_kernel_mode(v, regs) )
+            goto fail;
+
         pv_cpuid(regs);
         break;
 
     default:
         goto fail;
     }
 
 #undef wr_ad
diff --git a/xen/include/asm-x86/domain.h b/xen/include/asm-x86/domain.h
index 5807a1f..f6a40eb 100644
--- a/xen/include/asm-x86/domain.h
+++ b/xen/include/asm-x86/domain.h
@@ -552,16 +552,19 @@ struct arch_vcpu
      * However, processor should not be able to touch eXtended states before
      * it explicitly enables it via xcr0.
      */
     uint64_t xcr0_accum;
     /* This variable determines whether nonlazy extended state has been used,
      * and thus should be saved/restored. */
     bool_t nonlazy_xstate_used;
 
+    /* Has the guest enabled CPUID faulting? */
+    bool cpuid_faulting;
+
     /*
      * The SMAP check policy when updating runstate_guest(v) and the
      * secondary system time.
      */
     smap_check_policy_t smap_check_policy;
 
     struct vmce vmce;
 
diff --git a/xen/include/asm-x86/hvm/hvm.h b/xen/include/asm-x86/hvm/hvm.h
index b1be6bd..7e7462e 100644
--- a/xen/include/asm-x86/hvm/hvm.h
+++ b/xen/include/asm-x86/hvm/hvm.h
@@ -409,16 +409,17 @@ bool hvm_set_guest_bndcfgs(struct vcpu *v, u64 val);
 #define has_viridian_apic_assist(d) \
     (is_viridian_domain(d) && (viridian_feature_mask(d) & HVMPV_apic_assist))
 
 void hvm_hypervisor_cpuid_leaf(uint32_t sub_idx,
                                uint32_t *eax, uint32_t *ebx,
                                uint32_t *ecx, uint32_t *edx);
 void hvm_cpuid(unsigned int input, unsigned int *eax, unsigned int *ebx,
                                    unsigned int *ecx, unsigned int *edx);
+bool hvm_check_cpuid_faulting(struct vcpu *v);
 void hvm_migrate_timers(struct vcpu *v);
 void hvm_do_resume(struct vcpu *v);
 void hvm_migrate_pirqs(struct vcpu *v);
 
 void hvm_inject_trap(const struct hvm_trap *trap);
 void hvm_inject_hw_exception(unsigned int trapnr, int errcode);
 void hvm_inject_page_fault(int errcode, unsigned long cr2);
 
-- 
2.10.1


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Re: [PATCH v5 2/2] x86/Intel: virtualize support for cpuid faulting

[Andrew Cooper]

On 20/10/16 14:44, Kyle Huey wrote:
> On HVM guests, the cpuid triggers a vm exit, so we can check the emulated
> faulting state in vmx_do_cpuid and hvmemul_cpuid. A new function,
> hvm_check_cpuid_fault will check if cpuid faulting is enabled and the CPL > 0.
> When it returns true, the cpuid handling functions will inject a GP(0). Notably
> explicit hardware support for faulting on cpuid is not necessary to emulate
> support for an HVM guest.
>
> On PV guests, hardware support is required so that userspace cpuid will trap
> to Xen. Xen already enables cpuid faulting on supported CPUs for pv guests (that
> aren't the control domain, see the comment in intel_ctxt_switch_levelling).
> Every PV guest cpuid will trap via a GP(0) to emulate_privileged_op (via
> do_general_protection). Once there we simply decline to emulate cpuid if the
> CPL > 0 and faulting is enabled, leaving the GP(0) for the guest kernel to
> handle.
>
> Signed-off-by: Kyle Huey <khuey@kylehuey.com>
> Reviewed-by: Kevin Tian <kevin.tian@intel.com>

Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

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Re: [PATCH v5] x86/Intel: virtualize support for cpuid faulting

[Wei Liu]

On Thu, Oct 20, 2016 at 06:44:26AM -0700, Kyle Huey wrote:
> rr (http://rr-project.org/), a Linux userspace record-and-replay reverse-
> execution debugger, would like to trap and emulate the CPUID instruction.
> This would allow us to a) mask away certain hardware features that rr does
> not support (e.g. RDRAND) and b) enable trace portability across machines
> by providing constant results. Patches for support in the Linux kernel are in
> flight, and we'd like to be able to use this feature on virtualized Linux
> instances as well.
> 
> Changes since v4:
> - Renamed cpuid_fault to cpuid_faulting across the patch.
> - Various style nits.
> - Added Reviewed-by Kevin Tian.
> 

Release-acked-by: Wei Liu <wei.liu2@citrix.com>

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Re: [PATCH v5] x86/Intel: virtualize support for cpuid faulting

[Andrew Cooper]

On 20/10/16 15:15, Wei Liu wrote:
> On Thu, Oct 20, 2016 at 06:44:26AM -0700, Kyle Huey wrote:
>> rr (http://rr-project.org/), a Linux userspace record-and-replay reverse-
>> execution debugger, would like to trap and emulate the CPUID instruction.
>> This would allow us to a) mask away certain hardware features that rr does
>> not support (e.g. RDRAND) and b) enable trace portability across machines
>> by providing constant results. Patches for support in the Linux kernel are in
>> flight, and we'd like to be able to use this feature on virtualized Linux
>> instances as well.
>>
>> Changes since v4:
>> - Renamed cpuid_fault to cpuid_faulting across the patch.
>> - Various style nits.
>> - Added Reviewed-by Kevin Tian.
>>
> Release-acked-by: Wei Liu <wei.liu2@citrix.com>

Committed, along with its regression test.

Thanks.

~Andrew

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Re: [PATCH v5] x86/Intel: virtualize support for cpuid faulting

[Kyle Huey]

On Thu, Oct 20, 2016 at 9:19 AM, Andrew Cooper
<andrew.cooper3@citrix.com> wrote:
> On 20/10/16 15:15, Wei Liu wrote:
>> On Thu, Oct 20, 2016 at 06:44:26AM -0700, Kyle Huey wrote:
>>> rr (http://rr-project.org/), a Linux userspace record-and-replay reverse-
>>> execution debugger, would like to trap and emulate the CPUID instruction.
>>> This would allow us to a) mask away certain hardware features that rr does
>>> not support (e.g. RDRAND) and b) enable trace portability across machines
>>> by providing constant results. Patches for support in the Linux kernel are in
>>> flight, and we'd like to be able to use this feature on virtualized Linux
>>> instances as well.
>>>
>>> Changes since v4:
>>> - Renamed cpuid_fault to cpuid_faulting across the patch.
>>> - Various style nits.
>>> - Added Reviewed-by Kevin Tian.
>>>
>> Release-acked-by: Wei Liu <wei.liu2@citrix.com>
>
> Committed, along with its regression test.
>
> Thanks.
>
> ~Andrew

Great!  Thanks for all your help.

- Kyle

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