[PATCH v2 2/8] x86/svm: silently drop writes to SYSCFG and related MSRs

[Roger Pau Monne <roger.pau@xxxxxxxxxx>]

The SYSCFG, TOP_MEM1 and TOP_MEM2 MSRs are currently exposed to guests
and writes are silently discarded. Make this explicit in the SVM code
now, and just return default constant values when attempting to read
any of the MSRs, while continuing to silently drop writes.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
Changes sincxe v1:
 - Return MtrrFixDramEn in MSR_K8_SYSCFG.
---
 xen/arch/x86/hvm/svm/svm.c | 21 +++++++++++++++++++++
 1 file changed, 21 insertions(+)

diff --git a/xen/arch/x86/hvm/svm/svm.c b/xen/arch/x86/hvm/svm/svm.c
index ca3bbfcbb3..2d0823e7e1 100644
--- a/xen/arch/x86/hvm/svm/svm.c
+++ b/xen/arch/x86/hvm/svm/svm.c
@@ -1917,6 +1917,21 @@ static int svm_msr_read_intercept(unsigned int msr, 
uint64_t *msr_content)
             goto gpf;
         break;
 
+    case MSR_K8_TOP_MEM1:
+    case MSR_K8_TOP_MEM2:
+        *msr_content = 0;
+        break;
+
+    case MSR_K8_SYSCFG:
+        /*
+         * Return MtrrFixDramEn: albeit the current emulated MTRR
+         * implementation doesn't support the Extended Type-Field Format having
+         * such bit set is common on AMD hardware and is harmless as long as
+         * MtrrFixDramModEn isn't set.
+         */
+        *msr_content = K8_MTRRFIXRANGE_DRAM_ENABLE;
+        break;
+
     case MSR_K8_VM_CR:
         *msr_content = 0;
         break;
@@ -2094,6 +2109,12 @@ static int svm_msr_write_intercept(unsigned int msr, 
uint64_t msr_content)
             goto gpf;
         break;
 
+    case MSR_K8_TOP_MEM1:
+    case MSR_K8_TOP_MEM2:
+    case MSR_K8_SYSCFG:
+        /* Drop writes. */
+        break;
+
     case MSR_K8_VM_CR:
         /* ignore write. handle all bits as read-only. */
         break;
-- 
2.28.0