Re: [PATCH] Print a user-friendly message on failed vmentry

Mohammed Gamal
2010-05-20T15:55:02Z

On Thu, May 20, 2010 at 6:53 PM, Avi Kivity <avi@redhat.com> wrote:
> On 05/20/2010 05:46 PM, Mohammed Gamal wrote:
>>
>> On Thu, May 20, 2010 at 5:37 PM, Chris Lalancette<clalance@redhat.com>
>>  wrote:
>>
>>>
>>> On 05/19/2010 05:16 PM, Mohammed Gamal wrote:
>>>
>>>>
>>>> This patch address bug report in
>>>> https://bugs.launchpad.net/qemu/+bug/530077.
>>>>
>>>> Failed vmentries were handled with handle_unhandled() which prints a
>>>> rather
>>>> unfriendly message to the user. This patch separates handling vmentry
>>>> failures
>>>> from unknown exit reasons and prints a friendly message to the user.
>>>>
>>>> Signed-off-by: Mohammed Gamal<m.gamal005@gmail.com>
>>>> ---
>>>>  qemu-kvm.c |   16 +++++++++++++++-
>>>>  1 files changed, 15 insertions(+), 1 deletions(-)
>>>>
>>>> diff --git a/qemu-kvm.c b/qemu-kvm.c
>>>> index 35a4c8a..deb4df8 100644
>>>> --- a/qemu-kvm.c
>>>> +++ b/qemu-kvm.c
>>>> @@ -106,6 +106,20 @@ static int handle_unhandled(uint64_t reason)
>>>>      return -EINVAL;
>>>>  }
>>>>
>>>> +static int handle_failed_vmentry(uint64_t reason)
>>>> +{
>>>> +    fprintf(stderr, "kvm: vm entry failed with error 0x%" PRIx64
>>>> "\n\n", reason);
>>>> +    fprintf(stderr, "If you're runnning a guest on an Intel machine, it
>>>> can be\n");
>>>> +    fprintf(stderr, "most-likely due to the guest going into an invalid
>>>> state\n");
>>>> +    fprintf(stderr, "for Intel VT. For example, the guest maybe running
>>>> in big\n");
>>>> +    fprintf(stderr, "real mode which is not supported by Intel
>>>> VT.\n\n");
>>>> +    fprintf(stderr, "You may want to try enabling real mode emulation
>>>> in KVM.\n");
>>>> +    fprintf(stderr, "To Enable it, you may run the following commands
>>>> as root:\n");
>>>> +    fprintf(stderr, "# rmmod kvm_intel\n");
>>>> +    fprintf(stderr, "# rmmod kvm\n");
>>>> +    fprintf(stderr, "# modprobe kvm_intel
>>>> emulate_invalid_guest_state=1\n");
>>>> +    return -EINVAL;
>>>> +}
>>>>
>>>
>>> The thing is, there are other valid reasons for vmentry failure.  A while
>>> ago I tracked
>>> down a bug in the Linux kernel that was causing us to vmenter with
>>> invalid segments;
>>> this message would have been very misleading in that case.  I think you'd
>>> have to do
>>> more complete analysis of the vmentry failure code to be more certain
>>> about the reason
>>> for failure.
>>>
>>>
>>
>> Your point is definitely valid, yet big real mode is usually the most
>> likely case, and that's why this message is shown. Note also that it
>> says it's _most likely_ a failure caused by an invalid guest state,
>> but it doesn't rule out all other reasons. And in any case, it'd be
>> better than just printing something along the lines of:
>> " kvm: unhandled exit 80000021
>>   kvm_run returned -22"
>>
>
> However, we're still giving bad advice.  Currently
> emulate_invalid_guest_state=1 is not going to work well (right?).  Once it
> does, we'll simply make it the default and the message will never appear.
>
I already added a warning in the second patch I sent.

> --
> Do not meddle in the internals of kernels, for they are subtle and quick to
> panic.
>
>