Legacy hypervisors (RHEL 5.0 and RHEL 5.1) do not handle guest writes to

cr4 gracefully. If a guest attempts to write a bit of cr4 that is

unsupported, then the HV is so offended it crashes the domain. While

later guest kernels (such as RHEL6) don't assume the HV supports all

features, they do expect nicer responses. That assumption introduced

code that probes whether or not xsave is supported early in the boot. So

now when attempting to boot a RHEL6 guest on RHEL5.0 or RHEL5.1 an early

crash will occur.

This patch is quite obviously an undesirable hack. The real fix for this

problem should be in the HV, and is, in later HVs. However, to support

running on old HVs, RHEL6 can take this small change. No impact will

occur for running on any RHEL HV (not even RHEL 5.5 supports xsave).

There is only potential for guest performance loss on upstream Xen.

---

 arch/x86/xen/enlighten.c |    1 +

 1 files changed, 1 insertions(+), 0 deletions(-)

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c

index 52f8e19..6db3d67 100644

--- a/arch/x86/xen/enlighten.c

+++ b/arch/x86/xen/enlighten.c

@@ -802,6 +802,7 @@ static void xen_write_cr4(unsigned long cr4)

 {

 	cr4 &= ~X86_CR4_PGE;

 	cr4 &= ~X86_CR4_PSE;

+	cr4 &= ~X86_CR4_OSXSAVE;

 	native_write_cr4(cr4);

 }

-- 

1.6.6.1