Re: [PATCH] x86, kvm: Clear paravirt_enabled on KVM guests for espfix32's benefit

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On Mon, Dec 8, 2014 at 7:45 AM, Konrad Rzeszutek Wilk
<konrad.wilk@xxxxxxxxxx> wrote:
> On Fri, Dec 05, 2014 at 07:03:28PM -0800, Andy Lutomirski wrote:
>> paravirt_enabled has the following effects:
>>
>>  - Disables the F00F bug workaround warning.  There is no F00F bug
>>    workaround any more because Linux's standard IDT handling already
>>    works around the F00F bug, but the warning still exists.  This
>>    is only cosmetic, and, in any event, there is no such thing as
>>    KVM on a CPU with the F00F bug.
>>
>>  - Disables 32-bit APM BIOS detection.  On a KVM paravirt system,
>>    there should be no APM BIOS anyway.
>>
>>  - Disables tboot.  I think that the tboot code should check the
>>    CPUID hypervisor bit directly if it matters.
>>
>>  - paravirt_enabled disables espfix32.  espfix32 should *not* be
>>    disabled under KVM paravirt.
>>
>> The last point is the purpose of this patch.  It fixes a leak of the
>> high 16 bits of the kernel stack address on 32-bit KVM paravirt
>> guests.
>>
>> While I'm at it, this removes pv_info setup from kvmclock.  That
>> code seems to serve no purpose.
>>
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
>
> Suggested-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

Sorry, meant to add that but forgot.  Too many patches late last week :(

>> ---
>>  arch/x86/kernel/kvm.c      | 9 ++++++++-
>>  arch/x86/kernel/kvmclock.c | 2 --
>>  2 files changed, 8 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>> index f6945bef2cd1..94f643484300 100644
>> --- a/arch/x86/kernel/kvm.c
>> +++ b/arch/x86/kernel/kvm.c
>> @@ -283,7 +283,14 @@ NOKPROBE_SYMBOL(do_async_page_fault);
>>  static void __init paravirt_ops_setup(void)
>>  {
>>       pv_info.name = "KVM";
>> -     pv_info.paravirt_enabled = 1;
>> +
>> +     /*
>> +      * KVM isn't paravirt in the sense of paravirt_enabled.  A KVM
>> +      * guest kernel works like a bare metal kernel with additional
>> +      * features, and paravirt_enabled is about features that are
>> +      * missing.
>> +      */
>> +     pv_info.paravirt_enabled = 0;
>>
>>       if (kvm_para_has_feature(KVM_FEATURE_NOP_IO_DELAY))
>>               pv_cpu_ops.io_delay = kvm_io_delay;
>> diff --git a/arch/x86/kernel/kvmclock.c b/arch/x86/kernel/kvmclock.c
>> index d9156ceecdff..d4d9a8ad7893 100644
>> --- a/arch/x86/kernel/kvmclock.c
>> +++ b/arch/x86/kernel/kvmclock.c
>> @@ -263,8 +263,6 @@ void __init kvmclock_init(void)
>>  #endif
>>       kvm_get_preset_lpj();
>>       clocksource_register_hz(&kvm_clock, NSEC_PER_SEC);
>> -     pv_info.paravirt_enabled = 1;
>> -     pv_info.name = "KVM";
>>
>>       if (kvm_para_has_feature(KVM_FEATURE_CLOCKSOURCE_STABLE_BIT))
>>               pvclock_set_flags(PVCLOCK_TSC_STABLE_BIT);
>> --
>> 1.9.3
>>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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