[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v4 0/5] [PATCH v3 0/5] Improve non-"safe" MSR access failure handling
On 03/12/2016 01:08 PM, Andy Lutomirski wrote: 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. With CONFIG_PARAVIRT=n, unchecked MSR failures OOPS and probably cause boot to fail if they happen before init starts. Neither behavior is very good, and it's particularly unfortunate that the behavior changes depending on CONFIG_PARAVIRT. In particular, KVM guests might be unwittingly depending on the PARAVIRT=y behavior because CONFIG_KVM_GUEST currently depends on CONFIG_PARAVIRT, and, because accesses in that case are completely unchecked, we wouldn't even see a warning. This series changes the native behavior, regardless of CONFIG_PARAVIRT. A non-"safe" MSR failure will give an informative warning once and will be fixed up -- native_read_msr will return zero, and both reads and writes will continue where they left off. If panic_on_oops is set, they will still OOPS and panic. By using the shiny new custom exception handler infrastructure, there should be no overhead on the success paths. I didn't change the behavior on Xen, but, with this series applied, it would be straightforward for the Xen maintainers to make the corresponding change -- knowledge of whether the access is "safe" is now propagated into the pvops. 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 :) There's also room to reduce the code size of the "safe" variants using custom exception handlers in the future. Changes from v3: - WARN_ONCE instead of WARN (Ingo) - In the warning text, s/unsafe/unchecked/ (Ingo, sort of) Changes from earlier versions: lots of changes! Andy Lutomirski (5): x86/paravirt: Add _safe to the read_msr and write_msr PV hooks x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops x86/paravirt: Add paravirt_{read,write}_msr x86/paravirt: Make "unsafe" MSR accesses unsafe even if PARAVIRT=y x86/msr: Set the return value to zero when native_rdmsr_safe fails arch/x86/include/asm/msr.h | 20 ++++++++++++---- arch/x86/include/asm/paravirt.h | 45 +++++++++++++++++++++-------------- arch/x86/include/asm/paravirt_types.h | 14 +++++++---- arch/x86/kernel/paravirt.c | 6 +++-- arch/x86/mm/extable.c | 33 +++++++++++++++++++++++++ arch/x86/xen/enlighten.c | 27 +++++++++++++++++++-- 6 files changed, 114 insertions(+), 31 deletions(-) I don't see any issues as far as Xen is concerned but let me run this through our nightly. I'll wait for the next version (which I think you might have based on the comments). Please copy me. -boris _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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