Re: [PATCH] KVM: x86: Remove user space triggerable MCE error message

[Jan Kiszka <jan.kiszka@web.de>]

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On 2011-01-17 08:31, Huang Ying wrote:
> On Mon, 2011-01-17 at 15:11 +0800, Jan Kiszka wrote:
>> On 2011-01-17 01:54, Huang Ying wrote:
>>> On Sat, 2011-01-15 at 17:00 +0800, Jan Kiszka wrote:
>>>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>>>
>>>> This case is a pure user space error we do not need to record. Moreover,
>>>> it can be misused to flood the kernel log. Remove it.
>>>
>>> I don't think this is a pure user space error.  This happens on real
>>> hardware too, if the Machine Check exception is raised during early boot
>>> stage or the second MC exception is raised before the first MC exception
>>> is processed/cleared.
>>>
>>> So I use printk here to help debugging these issues.
>>>
>>> To avoid flooding the kernel log, we can use ratelimit.
>>
>> With user space I meant qemu, and maybe "error" was the wrong term. This
>> code path is only triggered if qemu decides to.
> 
> Not only decided by qemu, but also decided by guest OS.  If guest OS
> does not clear the MSR or guest OS does not set the X86_CR4_MCE bit in
> the cr4, the triple fault will be triggered.

Right, but qemu has full access to the state and can detect this.

> 
>> And there you may also
>> print this event (and you already do).
> 
> Sorry, which print do you mean?  I can not find similar print in user
> space.

Err, it's not yet there for the kvm case, I was starring at the tcg code
path.

> 
>> Another reason to not rely on catching this case here: KVM_X86_SET_MCE
>> is obsolete on current kernels. Qemu will use a combination of
>> KVM_SET_MSRS and KVM_SET_VCPU_EVENTS in the future, only falling back to
>> this interface on pre-vcpu-events kernels. Then you need to debug this
>> in user space anyway as the triple fault will no longer make it to the
>> kernel.
> 
> OK.  Then, I think it will be helpful for debugging if we can print
> something like this in user space implementation.

qemu_log_mask(CPU_LOG_RESET, "Triple fault\n");

(and there will be a message on the monitor if raised manually)

Jan


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