[Qemu-devel] [PATCH v3 1/4] x86: Allow physical address bits to be set

[Dr. David Alan Gilbert (git)]

From: "Dr. David Alan Gilbert" <address@hidden>

Currently QEMU sets the x86 number of physical address bits to the
magic number 40.  This is only correct on some small AMD systems;
Intel systems tend to have 36, 39, 46 bits, and large AMD systems
tend to have 48.

Having the value different from your actual hardware is detectable
by the guest and in principal can cause problems;
The current limit of 40 stops TB VMs being created by those lucky
enough to have that much.

This patch lets you set the physical bits by a cpu property but
defaults to the same existing magic 40.

I've removed the ancient warning about the 42 bit limit in exec.c;
I can't find that limit in there and no one else seems to know where
it is.

We use a magic value of 9999 as the property default so that we can
later distinguish between the default and a user set value.

Signed-off-by: Dr. David Alan Gilbert <address@hidden>
---
 target-i386/cpu.c | 43 ++++++++++++++++++++++++++++++++++---------
 target-i386/cpu.h |  3 +++
 2 files changed, 37 insertions(+), 9 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 3bd3cfc..74d53c5 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2602,17 +2602,13 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
         break;
     case 0x80000008:
         /* virtual & phys address size in low 2 bytes. */
-/* XXX: This value must match the one used in the MMU code. */
         if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
-            /* 64 bit processor */
-/* XXX: The physical address space is limited to 42 bits in exec.c. */
-            *eax = 0x00003028; /* 48 bits virtual, 40 bits physical */
+            /* 64 bit processor, 48 bits virtual, configurable
+             * physical bits.
+             */
+            *eax = 0x00003000 + cpu->phys_bits;
         } else {
-            if (env->features[FEAT_1_EDX] & CPUID_PSE36) {
-                *eax = 0x00000024; /* 36 bits physical */
-            } else {
-                *eax = 0x00000020; /* 32 bits physical */
-            }
+            *eax = cpu->phys_bits;
         }
         *ebx = 0;
         *ecx = 0;
@@ -2956,7 +2952,35 @@ static void x86_cpu_realizefn(DeviceState *dev, Error 
**errp)
            & CPUID_EXT2_AMD_ALIASES);
     }
 
+    if (env->features[FEAT_8000_0001_EDX] & CPUID_EXT2_LM) {
+        /* 9999 is a special meaning 'use the old default',
+         */
+        if (cpu->phys_bits == 9999) {
+            /* this must match the PHYS_ADDR_MASK in cpu.h */
+            cpu->phys_bits = 40;
+        }
+        if (cpu->phys_bits > TARGET_PHYS_ADDR_SPACE_BITS ||
+            cpu->phys_bits < 32) {
+            error_setg(errp, "phys_bits should be between 32 and %u or 0 to"
+                             " use host size (but is %u)",
+                             TARGET_PHYS_ADDR_SPACE_BITS, cpu->phys_bits);
+            return;
+        }
+    } else {
+        /* For 32 bit systems don't use the user set value, but keep
+         * phys_bits consistent with what we tell the guest.
+         */
+        if (cpu->phys_bits != 9999) {
+            error_setg(errp, "phys_bits is not user-configurable in 32 bit");
+            return;
+        }
 
+        if (env->features[FEAT_1_EDX] & CPUID_PSE36) {
+            cpu->phys_bits = 36;
+        } else {
+            cpu->phys_bits = 32;
+        }
+    }
     cpu_exec_init(cs, &error_abort);
 
     if (tcg_enabled()) {
@@ -3257,6 +3281,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, true),
     DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
+    DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 9999),
     DEFINE_PROP_UINT32("level", X86CPU, env.cpuid_level, 0),
     DEFINE_PROP_UINT32("xlevel", X86CPU, env.cpuid_xlevel, 0),
     DEFINE_PROP_UINT32("xlevel2", X86CPU, env.cpuid_xlevel2, 0),
diff --git a/target-i386/cpu.h b/target-i386/cpu.h
index 474b0b9..221b1a2 100644
--- a/target-i386/cpu.h
+++ b/target-i386/cpu.h
@@ -1181,6 +1181,9 @@ struct X86CPU {
     /* Compatibility bits for old machine types: */
     bool enable_cpuid_0xb;
 
+    /* Number of physical address bits supported */
+    uint32_t phys_bits;
+
     /* in order to simplify APIC support, we leave this pointer to the
        user */
     struct DeviceState *apic_state;
-- 
2.7.4