[Xen-devel] [PATCH] xen/vPMU: Do not clobber IA32_MISC_ENABLE

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The VMX RDMSR intercept for MSR_IA32_MISC_ENABLE falls through into
vpmu_do_rdmsr(), so that core2_vpmu_do_rdmsr() may play with the PTS and PEBS
UNAVAIL bits.

Some 64bit Windows include IA32_MISC_ENABLE in the set of items checked by
PatchGuard, and will suffer a BSOD 0x109 CRITICAL_STRUCTURE_CORRUPTION if the
contents change on migrate.

The vPMU infrastructure should not clobber IA32_MISC_ENABLE at all.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>

This appears to have been broken since the vPMU code was first introduced.  It
appears to have lurked this log due to a hole (now fixed) in XenServers
upgrade testing.  The BSODs occur ~80% of the time on Win 8 thru 10, but
appear very hard to provoke on Windows 7.

This MSR still leaks mostly host state through into the guest.  Therefore
migration of windows is still liable to crash if moving between two
non-identical servers.  I need to get proper MSR levelling sorted before this
issue can be resolved fully.
---
 xen/arch/x86/cpu/vpmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/xen/arch/x86/cpu/vpmu.c b/xen/arch/x86/cpu/vpmu.c
index 237b5ff..2f9ddf6 100644
--- a/xen/arch/x86/cpu/vpmu.c
+++ b/xen/arch/x86/cpu/vpmu.c
@@ -169,7 +169,7 @@ int vpmu_do_msr(unsigned int msr, uint64_t *msr_content,
     return ret;
 
  nop:
-    if ( !is_write )
+    if ( !is_write && (msr != MSR_IA32_MISC_ENABLE) )
         *msr_content = 0;
 
     return 0;
-- 
2.1.4


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