[Xen-devel] [PATCH V2 2/2] xen: Introduce VM_EVENT_FLAG_SET_REGISTERS

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

A previous version of this patch dealing with support for skipping
the current instruction when a vm_event response requested it
computed the instruction length in the hypervisor, adding non-trivial
code dependencies. This patch allows a userspace vm_event client to
simply request that the guest's EIP is set to an arbitary value,
computed by the introspection application. In the future, other
registers can also be set via a vm_event reply by using this flag.
The VCPU needs to be paused for this flag to take effect.

Signed-off-by: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>

---
Changes since V1:
 - Renamed the patch (VM_EVENT_FLAG_SET_EIP ->
   VM_EVENT_FLAG_SET_REGISTERS).
 - As suggested by Tamas Lengyel, EIP is now being set via a dedicated
   generic vm_event_set_registers() function that can be extended to
   set other registers in the future.
---
 xen/arch/x86/vm_event.c        |    5 +++++
 xen/common/vm_event.c          |    3 +++
 xen/include/asm-arm/vm_event.h |    6 ++++++
 xen/include/asm-x86/vm_event.h |    2 ++
 xen/include/public/vm_event.h  |    6 ++++++
 5 files changed, 22 insertions(+)

diff --git a/xen/arch/x86/vm_event.c b/xen/arch/x86/vm_event.c
index e4e0aa4..a59ba79 100644
--- a/xen/arch/x86/vm_event.c
+++ b/xen/arch/x86/vm_event.c
@@ -95,6 +95,11 @@ void vm_event_register_write_resume(struct vcpu *v, 
vm_event_response_t *rsp)
     }
 }
 
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
+{
+    v->arch.user_regs.eip = rsp->data.regs.x86.rip;
+}
+
 /*
  * Local variables:
  * mode: C
diff --git a/xen/common/vm_event.c b/xen/common/vm_event.c
index ef84b0f..e1f9580 100644
--- a/xen/common/vm_event.c
+++ b/xen/common/vm_event.c
@@ -417,6 +417,9 @@ void vm_event_resume(struct domain *d, struct 
vm_event_domain *ved)
 
         if ( rsp.flags & VM_EVENT_FLAG_VCPU_PAUSED )
         {
+            if ( rsp.flags & VM_EVENT_FLAG_SET_REGISTERS )
+                vm_event_set_registers(v, &rsp);
+
             if ( rsp.flags & VM_EVENT_FLAG_TOGGLE_SINGLESTEP )
                 vm_event_toggle_singlestep(d, v);
 
diff --git a/xen/include/asm-arm/vm_event.h b/xen/include/asm-arm/vm_event.h
index 976fdf1..4d0fbf7 100644
--- a/xen/include/asm-arm/vm_event.h
+++ b/xen/include/asm-arm/vm_event.h
@@ -47,4 +47,10 @@ void vm_event_register_write_resume(struct vcpu *v, 
vm_event_response_t *rsp)
     /* Not supported on ARM. */
 }
 
+static inline
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp)
+{
+    /* Not supported on ARM. */
+}
+
 #endif /* __ASM_ARM_VM_EVENT_H__ */
diff --git a/xen/include/asm-x86/vm_event.h b/xen/include/asm-x86/vm_event.h
index 2ff2cab..5aff834 100644
--- a/xen/include/asm-x86/vm_event.h
+++ b/xen/include/asm-x86/vm_event.h
@@ -42,4 +42,6 @@ void vm_event_toggle_singlestep(struct domain *d, struct vcpu 
*v);
 
 void vm_event_register_write_resume(struct vcpu *v, vm_event_response_t *rsp);
 
+void vm_event_set_registers(struct vcpu *v, vm_event_response_t *rsp);
+
 #endif /* __ASM_X86_VM_EVENT_H__ */
diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
index ff2f217..51539af 100644
--- a/xen/include/public/vm_event.h
+++ b/xen/include/public/vm_event.h
@@ -89,6 +89,12 @@
  * by the altp2m_idx response field if possible.
  */
 #define VM_EVENT_FLAG_ALTERNATE_P2M      (1 << 7)
+/*
+ * Set the vCPU registers to the values in the  vm_event response.
+ * Currently only applies to EIP.
+ * Requires the vCPU to be paused already (synchronous events only).
+ */
+#define VM_EVENT_FLAG_SET_REGISTERS      (1 << 8)
 
 /*
  * Reasons for the vm event request
-- 
1.7.9.5


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