Changeset 45892 in vbox for trunk/src/VBox/VMM

Timestamp:

May 3, 2013 11:44:10 AM (12 years ago)

Author:

vboxsync

Message:

VMM/HMVMXR0: Some clarification regarding the unusable bit.

File:

• trunk/src/VBox/VMM/VMMR0/HMVMXR0.cpp (modified) (9 diffs)

trunk/src/VBox/VMM/VMMR0/HMVMXR0.cpp

@@ -3152 +3152 @@
     Assert(pVCpu->hm.s.vmx.fUpdatedGuestState & HMVMX_UPDATED_GUEST_CR0);
     Assert(pVCpu->hm.s.vmx.fUpdatedGuestState & HMVMX_UPDATED_GUEST_RFLAGS);
+
+    /* NOTE: The reason we check for attribute value 0 and not just the unusable bit here is because hmR0VmxWriteSegmentReg()
+     * only updates the VMCS bits with the unusable bit and doesn't change the guest-context value. */
     if (   !pVM->hm.s.vmx.fUnrestrictedGuest
         && (   !CPUMIsGuestInRealModeEx(pCtx)
@@ -3165 +3168 @@
         Assert(   !(pCtx->cs.u32Limit & 0xfff00000)
                || (pCtx->cs.Attr.n.u1Granularity));
-        Assert(pCtx->cs.Attr.u && pCtx->cs.Attr.u != HMVMX_SEL_UNUSABLE);  /* CS cannot be loaded with NULL in protected mode. */
+        /* CS cannot be loaded with NULL in protected mode. */
+        Assert(pCtx->cs.Attr.u && !(pCtx->cs.Attr.u & HMVMX_SEL_UNUSABLE)); /** @todo is this really true even for 64-bit CS?!? */
         if (pCtx->cs.Attr.n.u4Type == 9 || pCtx->cs.Attr.n.u4Type == 11)
             Assert(pCtx->cs.Attr.n.u2Dpl == pCtx->ss.Attr.n.u2Dpl);
@@ -3181 +3185 @@
             Assert(!pCtx->ss.Attr.n.u2Dpl);
         }
-        if (pCtx->ss.Attr.u && pCtx->ss.Attr.u != HMVMX_SEL_UNUSABLE)
+        if (pCtx->ss.Attr.u && !(pCtx->ss.Attr.u & HMVMX_SEL_UNUSABLE))
         {
             Assert((pCtx->ss.Sel & X86_SEL_RPL) == (pCtx->cs.Sel & X86_SEL_RPL));
@@ -3194 +3198 @@
         }
         /* DS, ES, FS, GS - only check for usable selectors, see hmR0VmxWriteSegmentReg(). */
-        if (pCtx->ds.Attr.u && pCtx->ds.Attr.u != HMVMX_SEL_UNUSABLE)
+        if (pCtx->ds.Attr.u && !(pCtx->ds.Attr.u & HMVMX_SEL_UNUSABLE))
         {
             Assert(pCtx->ds.Attr.n.u4Type & X86_SEL_TYPE_ACCESSED);
@@ -3208 +3212 @@
                    || (pCtx->ds.Attr.n.u4Type & X86_SEL_TYPE_READ));
         }
-        if (pCtx->es.Attr.u && pCtx->es.Attr.u != HMVMX_SEL_UNUSABLE)
+        if (pCtx->es.Attr.u && !(pCtx->es.Attr.u & HMVMX_SEL_UNUSABLE))
         {
             Assert(pCtx->es.Attr.n.u4Type & X86_SEL_TYPE_ACCESSED);
@@ -3222 +3226 @@
                    || (pCtx->es.Attr.n.u4Type & X86_SEL_TYPE_READ));
         }
-        if (pCtx->fs.Attr.u && pCtx->fs.Attr.u != HMVMX_SEL_UNUSABLE)
+        if (pCtx->fs.Attr.u && !(pCtx->fs.Attr.u & HMVMX_SEL_UNUSABLE))
         {
             Assert(pCtx->fs.Attr.n.u4Type & X86_SEL_TYPE_ACCESSED);
@@ -3236 +3240 @@
                    || (pCtx->fs.Attr.n.u4Type & X86_SEL_TYPE_READ));
         }
-        if (pCtx->gs.Attr.u && pCtx->gs.Attr.u != HMVMX_SEL_UNUSABLE)
+        if (pCtx->gs.Attr.u && !(pCtx->gs.Attr.u & HMVMX_SEL_UNUSABLE))
         {
             Assert(pCtx->gs.Attr.n.u4Type & X86_SEL_TYPE_ACCESSED);
@@ -3355 +3359 @@
 
     /* Validate segment access rights. Refer to Intel spec. "26.3.1.2 Checks on Guest Segment Registers". */
-    AssertMsg((u32Access == HMVMX_SEL_UNUSABLE) || (u32Access & X86_SEL_TYPE_ACCESSED),
+    AssertMsg((u32Access & HMVMX_SEL_UNUSABLE) || (u32Access & X86_SEL_TYPE_ACCESSED),
               ("Access bit not set for usable segment. idx=%#x sel=%#x attr %#x\n", idxBase, pSelReg, pSelReg->Attr.u));
 
@@ -5152 +5156 @@
 
     /*
-     * If VT-x marks the segment as unusable, the rest of the attributes are undefined.
+     * If VT-x marks the segment as unusable, the rest of the attributes are undefined with certain exceptions (some bits in
+     * CS, SS). Regardless, we have to clear the bits here and only retain the unusable bit because the unusable bit is specific
+     * to VT-x, everyone else relies on the attribute being zero and have no clue what the unusable bit is.
+     *
      * See Intel spec. 27.3.2 "Saving Segment Registers and Descriptor-Table Registers".
      */
     if (pSelReg->Attr.u & HMVMX_SEL_UNUSABLE)
     {
-        Assert(idxSel != VMX_VMCS16_GUEST_FIELD_TR);
-        /** @todo r=ramshankar: This can't be right for CS, SS which have exceptions for
-         *        certain bits, they're not all undefined. Consider ORing
-         *        HMVMX_SEL_UNUSABLE instead? */
+        Assert(idxSel != VMX_VMCS16_GUEST_FIELD_TR);          /* TR is the only selector that can never be unusable. */
         pSelReg->Attr.u = HMVMX_SEL_UNUSABLE;
     }