Bug#592428: Fix 2.6.32 XEN guest on old buggy RHEL5/EC2 hypervisor (XSAVE)
Package: linux-2.6
Severity: normal
Tags: patch
Would it be possible to apply the attached Fedora/Ubuntu kernel patch
to Debian as well? The Fedora link is:
http://cvs.fedoraproject.org/viewvc/F-13/kernel/fix_xen_guest_on_old_EC2.patch
And the Ubuntu link:
http://kernel.ubuntu.com/git?p=rtg/ubuntu-maverick.git;a=commit;h=1a30f99
As far as I can tell, no released version of Xen currently supports
XSAVE, so this change is effectively a NOP on all newer hypervisors, but
it allows functionality on older hypervisors (such as RHEL5, or when
running on Amazon's EC2 service).
In particular, I'm trying to write a script that packages up a vmlinuz
and initrd.gz from the Debian-Installer to allow them to be easily run
unmodified in an Amazon EC2 VM (now that Amazon supports using your own
custom kernel).
Cheers,
Kyle Moffett
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
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