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Re: [Qemu-devel] [PATCH] allow more than 1T in KVM x86 guest

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] allow more than 1T in KVM x86 guest
Date: Wed, 15 Sep 2010 13:26:15 -0500
User-agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv: Gecko/20100826 Lightning/1.0b1 Thunderbird/3.0.7

On 09/15/2010 12:13 PM, Andrea Arcangeli wrote:
Subject: allow more than 1T in KVM x86 guest

From: Andrea Arcangeli<address@hidden>

When host supports 48 bits of physical address reflect that in the guest cpuid
to allow the guest to use more than 1TB of RAM.

The migration code should probably be updated accordingly checking if the size
of the guest ram is bigger than the migration target cpuid 0x80000008 limit and
failing migration in that case. (not a real practical issue, I don't
see many people migrating>1T guests yet :)

The comment below refers to a 42 bit limit on exec.c, but I didn't identify
what the comment refers to yet. At least now guest should be able to use 4TB.


#ifdef TARGET_X86_64
/* ??? This is really 48 bits, sign-extended, but the only thing
   accessible to userland with bit 48 set is the VSYSCALL, and that
   is handled via other mechanisms.  */

The macros are then used in exec.c


Anthony Liguori

Signed-off-by: Andrea Arcangeli<address@hidden>

diff --git a/target-i386/cpuid.c b/target-i386/cpuid.c
index d63fdcb..462e709 100644
--- a/target-i386/cpuid.c
+++ b/target-i386/cpuid.c
@@ -1189,6 +1189,12 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, 
uint32_t count,
              /* 64 bit processor */
  /* XXX: The physical address space is limited to 42 bits in exec.c. */
              *eax = 0x00003028;        /* 48 bits virtual, 40 bits physical */
+            if (kvm_enabled()) {
+                uint32_t _eax;
+                host_cpuid(0x80000000, 0,&_eax, NULL, NULL, NULL);
+                if (_eax>= 0x80000008)
+                    host_cpuid(0x80000008, 0, eax, NULL, NULL, NULL);
+            }
          } else {
              if (env->cpuid_features&  CPUID_PSE36)
                  *eax = 0x00000024; /* 36 bits physical */

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