[Qemu-devel] [PATCH v4 5/5] x86: Set physical address bits based on host

["Dr. David Alan Gilbert (git)" <dgilbert@redhat.com>]

From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>

Add the host-phys-bits boolean property, if true, take phys-bits
from the hosts physical bits value, overriding either the default
or the user specified value.

We can also use the value we read from the host to check the users
explicitly set value and warn them if it doesn't match.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 target-i386/cpu.c | 73 ++++++++++++++++++++++++++++++++++++++++++++++++-------
 target-i386/cpu.h |  3 +++
 2 files changed, 67 insertions(+), 9 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 641c38b..72ca7fb 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2885,6 +2885,31 @@ static void x86_cpu_apic_realize(X86CPU *cpu, Error **errp)
 }
 #endif
 
+/* Note: Only safe for use on x86(-64) hosts */
+static uint32_t x86_host_phys_bits(void)
+{
+    uint32_t eax;
+    uint32_t host_phys_bits;
+
+    host_cpuid(0x80000000, 0, &eax, NULL, NULL, NULL);
+    if (eax >= 0x80000008) {
+        host_cpuid(0x80000008, 0, &eax, NULL, NULL, NULL);
+        /* Note: According to AMD doc 25481 rev 2.34 they have a field
+         * at 23:16 that can specify a maximum physical address bits for
+         * the guest that can override this value; but I've not seen
+         * anything with that set.
+         */
+        host_phys_bits = eax & 0xff;
+    } else {
+        /* It's an odd 64 bit machine that doesn't have the leaf for
+         * physical address bits; fall back to 36 that's most older
+         * Intel.
+         */
+        host_phys_bits = 36;
+    }
+
+    return host_phys_bits;
+}
 
 #define IS_INTEL_CPU(env) ((env)->cpuid_vendor1 == CPUID_VENDOR_INTEL_1 && \
                            (env)->cpuid_vendor2 == CPUID_VENDOR_INTEL_2 && \
@@ -2952,28 +2977,57 @@ static void x86_cpu_realizefn(DeviceState *dev, Error **errp)
            & CPUID_EXT2_AMD_ALIASES);
     }
 
+    /* For 64bit systems think about the number of physical bits to present.
+     * ideally this should be the same as the host; anything other than matching
+     * the host can cause incorrect guest behaviour.
+     * QEMU used to pick the magic value of 40 bits that corresponds to
+     * consumer AMD devices but nothing else.
+     */
+    if (cpu->host_phys_bits && !kvm_enabled()) {
+        error_setg(errp, "phys-bits can not be read from the host in"
+                         " TCG mode");
+        return;
+    }
+
     if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
-        /* 0 is a special meaning 'use the old default', which matches
-         * the value used by TCG (40).
-         */
-        if (cpu->phys_bits == 0) {
-            cpu->phys_bits = TCG_PHYS_ADDR_BITS;
-        }
         if (kvm_enabled()) {
-            if (cpu->phys_bits > TARGET_PHYS_ADDR_SPACE_BITS ||
-                cpu->phys_bits < 32) {
+            uint32_t host_phys_bits = x86_host_phys_bits();
+            static bool warned;
+
+            if (cpu->host_phys_bits) {
+                /* The user asked for us to use the host physical bits */
+                cpu->phys_bits = host_phys_bits;
+            }
+
+            /* Print a warning if the user set it to a value that's not the
+             * host value.
+             */
+            if (cpu->phys_bits != host_phys_bits && cpu->phys_bits != 0 &&
+                !warned) {
+                error_report("Warning: Host physical bits (%u)"
+                                 " does not match phys-bits property (%u)",
+                                 host_phys_bits, cpu->phys_bits);
+                warned = true;
+            }
+
+            if (cpu->phys_bits &&
+                (cpu->phys_bits > TARGET_PHYS_ADDR_SPACE_BITS ||
+                cpu->phys_bits < 32)) {
                 error_setg(errp, "phys-bits should be between 32 and %u "
                                  " (but is %u)",
                                  TARGET_PHYS_ADDR_SPACE_BITS, cpu->phys_bits);
                 return;
             }
         } else {
-            if (cpu->phys_bits != TCG_PHYS_ADDR_BITS) {
+            if (cpu->phys_bits && cpu->phys_bits != TCG_PHYS_ADDR_BITS) {
                 error_setg(errp, "TCG only supports phys-bits=%u",
                                   TCG_PHYS_ADDR_BITS);
                 return;
             }
         }
+        if (cpu->phys_bits == 0 && !cpu->host_phys_bits) {
+            cpu->phys_bits = TCG_PHYS_ADDR_BITS;
+        }
     } else {
         /* For 32 bit systems don't use the user set value, but keep
          * phys_bits consistent with what we tell the guest.
@@ -3290,6 +3344,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
     DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),
+    DEFINE_PROP_BOOL("host-phys-bits", X86CPU, host_phys_bits, false),
     DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),
     DEFINE_PROP_UINT32("level", X86CPU, env.cpuid_level, 0),
     DEFINE_PROP_UINT32("xlevel", X86CPU, env.cpuid_xlevel, 0),
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index 9d79146..3c4e64a 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -1184,6 +1184,9 @@ struct X86CPU {
     /* if true fill the top bits of the MTRR_PHYSMASKn variable range */
     bool fill_mtrr_mask;
 
+    /* if true override the phys_bits value with a value read from the host */
+    bool host_phys_bits;
+
     /* Number of physical address bits supported */
     uint32_t phys_bits;
 
-- 
2.7.4