[Qemu-devel] allow sysenter on 32bit guests running on vmx host

[Andrea Arcangeli <aarcange@redhat.com>]

From: Andrea Arcangeli <aarcange@redhat.com>

model=2 is not existent when vendor is intel and an errata of P6 says
that any model <= 2 when family is 6 lack sap feature, so windows and
linux 32bit guests disable sap in software and slowdown for no good
reason when running inside kvm on intel CPU.

Fix is to set model = 3 so it'll be the duron cpu when kvm runs on amd bare
metal (not anymore athlon but userland doesn't see the difference) and it'll be
PII that has sysenter functional like all cpus that run KVM have too when on
intel bare metal.

Patch will follow, workaround without this fix is -cpu qemu64,model=3.

There is a bug in skype that it checks if the sep feature is set to
run sysenter on intel chip inside its binary without passing through
ntdll, without verifying the model was <= 2 like windows does, so
windows forbids sysenter but skype calls it anyway and crashes (this
is why recent skype stopped working on KVM). This is skype bug and it
would trigger on real P6 hardware too (but nobody tests skype on
P6). qemu64,-sep fixed skype too. But because every time we have an
intel cpu running kvm, we also have sysenter functioning, we boots the
model to 3 so windows 32bit (and linux32bit) gets a performance boost
with sysenter too.

Signed-off-by: Andrea Arcangeli <aarcange@redhat.com>
---

diff --git a/target-i386/helper.c b/target-i386/helper.c
index 8a76abd..636e113 100644
--- a/target-i386/helper.c
+++ b/target-i386/helper.c
@@ -139,7 +139,8 @@ static x86_def_t x86_defs[] = {
         .vendor2 = CPUID_VENDOR_AMD_2,
         .vendor3 = CPUID_VENDOR_AMD_3,
         .family = 6,
-        .model = 2,
+        /* AMD Duron || PII Intel with sep capbility - P6 has no sep */
+        .model = 3,
         .stepping = 3,
         .features = PPRO_FEATURES | 
         /* these features are needed for Win64 and aren't fully implemented */