[Qemu-devel] [PATCH 4/5] x86: Allow physical address bits to be set

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

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

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.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
---
 target-i386/cpu.c | 8 +++++---
 target-i386/cpu.h | 3 +++
 2 files changed, 8 insertions(+), 3 deletions(-)

diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 4111240..c3bbf8e 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -2606,9 +2606,10 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
         /* 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 */
@@ -3273,6 +3274,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
     DEFINE_PROP_BOOL("fill-mtrr-mask", X86CPU, fill_mtrr_mask, true),
+    DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 40),
     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 13d8501..fccba72 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;
 
+    /* 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