[Xen-devel] [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops

[Andy Lutomirski <luto@xxxxxxxxxx>]

Setting CONFIG_PARAVIRT=y has an unintended side effect: it silently
turns all rdmsr and wrmsr operations into the safe variants without
any checks that the operations actually succeed.

This is IMO awful: it papers over bugs.  In particular, KVM gueests
might be unwittingly depending on this behavior because
CONFIG_KVM_GUEST currently depends on CONFIG_PARAVIRT.  I'm not
aware of any such problems, but applying this series would be a good
way to shake them out.

Fix it so that the MSR operations work the same on CONFIG_PARAVIRT=n
and CONFIG_PARAVIRT=y as long as Xen isn't being used.  The Xen
maintainers are welcome to make a similar change on top of this.

Since there's plenty of time before the next merge window, I think
we should apply and fix anything that breaks.

Doing this is probably a prerequisite to sanely decoupling
CONFIG_KVM_GUEST and CONFIG_PARAVIRT, which would probably make
Arjan and the rest of the Clear Containers people happy :)

Andy Lutomirski (3):
  x86/paravirt: Add _safe to the read_msr and write_msr PV hooks
  x86/paravirt: Add paravirt_{read,write}_msr
  x86/paravirt: Make "unsafe" MSR accesses unsafe even if PARAVIRT=y

 arch/x86/include/asm/paravirt.h       | 45 +++++++++++++++++++++--------------
 arch/x86/include/asm/paravirt_types.h | 12 +++++++---
 arch/x86/kernel/paravirt.c            |  6 +++--
 arch/x86/xen/enlighten.c              | 27 +++++++++++++++++++--
 4 files changed, 65 insertions(+), 25 deletions(-)

-- 
2.4.3


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