[Xen-devel] [PATCH for-next] x86/svm: Correct vm_event API for descriptor accesses

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

c/s d0a699a389f1 "x86/monitor: add support for descriptor access events"
introduced logic looking for what appeared to be exitinfo (not that this
exists in SVM - exitinfo1 or 2 do), but actually passed the exit IDT vectoring
information.  There is never any IDT vectoring involved in these intercepts so
the value passed is always zero.

In fact, SVM doesn't provide any information, even in exitinfo1 and 2.  Note
the error in the public API and state that this field is always 0, and drop
the SVM logic in hvm_monitor_descriptor_access().

In the SVM vmexit handler itself, optimise the switch statement by observing
that there is a linear transformation between the SVM exit_reason and
VM_EVENT_DESC_* values.  (Bloat-o-meter reports 6028 => 5877 for a saving of
151 bytes).

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>
CC: Tamas K Lengyel <tamas@xxxxxxxxxxxxx>
CC: Alexandru Isaila <aisaila@xxxxxxxxxxxxxxx>
CC: Petre Pircalabu <ppircalabu@xxxxxxxxxxxxxxx>
CC: Adrian Pop <apop@xxxxxxxxxxxxxxx>

Adrian: Do you recall what information you were attempting to forward from the
VMCB?  I can't locate anything which would plausibly be interesting.

This is part of a longer cleanup series I gathered in the wake of the task
switch issues, but it is pulled out ahead due to its interaction with the
public interface.
---
 xen/arch/x86/hvm/monitor.c    |  4 ----
 xen/arch/x86/hvm/svm/svm.c    | 37 +++++++++++++++++--------------------
 xen/include/public/vm_event.h |  4 ++--
 3 files changed, 19 insertions(+), 26 deletions(-)

diff --git a/xen/arch/x86/hvm/monitor.c b/xen/arch/x86/hvm/monitor.c
index 7fb1e2c04e..1f23fe25e8 100644
--- a/xen/arch/x86/hvm/monitor.c
+++ b/xen/arch/x86/hvm/monitor.c
@@ -113,10 +113,6 @@ void hvm_monitor_descriptor_access(uint64_t exit_info,
         req.u.desc_access.arch.vmx.instr_info = exit_info;
         req.u.desc_access.arch.vmx.exit_qualification = vmx_exit_qualification;
     }
-    else
-    {
-        req.u.desc_access.arch.svm.exitinfo = exit_info;
-    }
 
     monitor_traps(current, true, &req);
 }
diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index 0fb1908c18..776cf11459 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -2980,29 +2980,26 @@ void svm_vmexit_handler(struct cpu_user_regs *regs)
         svm_vmexit_do_pause(regs);
         break;
 
-    case VMEXIT_IDTR_READ:
-    case VMEXIT_IDTR_WRITE:
-        hvm_descriptor_access_intercept(vmcb->exitintinfo.bytes, 0,
-            VM_EVENT_DESC_IDTR, exit_reason == VMEXIT_IDTR_WRITE);
-        break;
-
-    case VMEXIT_GDTR_READ:
-    case VMEXIT_GDTR_WRITE:
-        hvm_descriptor_access_intercept(vmcb->exitintinfo.bytes, 0,
-            VM_EVENT_DESC_GDTR, exit_reason == VMEXIT_GDTR_WRITE);
-        break;
+    case VMEXIT_IDTR_READ ... VMEXIT_TR_WRITE:
+    {
+        /*
+         * Consecutive block of 8 exit codes (sadly not aligned).  Top bit
+         * indicates write (vs read), bottom 2 bits map linearly to
+         * VM_EVENT_DESC_* values.
+         */
+#define E2D(e)      ((((e)         - VMEXIT_IDTR_READ) & 3) + 1)
+        bool write = ((exit_reason - VMEXIT_IDTR_READ) & 4);
+        unsigned int desc = E2D(exit_reason);
 
-    case VMEXIT_LDTR_READ:
-    case VMEXIT_LDTR_WRITE:
-        hvm_descriptor_access_intercept(vmcb->exitintinfo.bytes, 0,
-            VM_EVENT_DESC_LDTR, exit_reason == VMEXIT_LDTR_WRITE);
-        break;
+        BUILD_BUG_ON(E2D(VMEXIT_IDTR_READ) != VM_EVENT_DESC_IDTR);
+        BUILD_BUG_ON(E2D(VMEXIT_GDTR_READ) != VM_EVENT_DESC_GDTR);
+        BUILD_BUG_ON(E2D(VMEXIT_LDTR_READ) != VM_EVENT_DESC_LDTR);
+        BUILD_BUG_ON(E2D(VMEXIT_TR_READ)   != VM_EVENT_DESC_TR);
+#undef E2D
 
-    case VMEXIT_TR_READ:
-    case VMEXIT_TR_WRITE:
-        hvm_descriptor_access_intercept(vmcb->exitintinfo.bytes, 0,
-            VM_EVENT_DESC_TR, exit_reason == VMEXIT_TR_WRITE);
+        hvm_descriptor_access_intercept(0, 0, desc, write);
         break;
+    }
 
     default:
     unexpected_exit_type:
diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
index 959083d8c4..d1b5c95f72 100644
--- a/xen/include/public/vm_event.h
+++ b/xen/include/public/vm_event.h
@@ -302,8 +302,8 @@ struct vm_event_desc_access {
             uint64_t exit_qualification; /* VMX: VMCS Exit Qualification */
         } vmx;
         struct {
-            uint64_t exitinfo;           /* SVM: VMCB EXITINFO */
-            uint64_t _pad2;
+            uint64_t exitinfo;           /* SVM: Always 0.  This field made */
+            uint64_t _pad2;              /* its way into the API by error.  */
         } svm;
     } arch;
     uint8_t descriptor;                  /* VM_EVENT_DESC_* */
-- 
2.11.0


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