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

Mohammed Gamal
2010-05-20T14:46:53Z

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"

> --
> Chris Lalancette
>