Re: [PATCH V4 6/6] x86/Hyper-V: Report crash data in die() when panic_on_oops is set

[Tianyu Lan <ltykernel@gmail.com>]

On 4/2/2020 2:53 AM, Wei Liu wrote:
> On Tue, Mar 31, 2020 at 10:26:06PM +0800, Tianyu Lan wrote:
>>
>>
>> On 3/31/2020 9:51 PM, Michael Kelley wrote:
>>> From: Tianyu Lan <Tianyu.Lan@microsoft.com> Sent: Tuesday, March 31, 2020 12:39 AM
>>>>
>>>> When oops happens with panic_on_oops unset, the oops
>>>> thread is killed by die() and system continues to run.
>>>> In such case, guest should not report crash register
>>>> data to host since system still runs. Fix it.
>>>>
>>>> Reviewed-by: Michael Kelley <mikelley@microsoft.com>
>>>> Signed-off-by: Tianyu Lan <Tianyu.Lan@microsoft.com>
>>>> ---
>>>> Change since v3:
>>>> 	Fix compile error
>>>> ---
>>>>    drivers/hv/vmbus_drv.c | 3 ++-
>>>>    1 file changed, 2 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/drivers/hv/vmbus_drv.c b/drivers/hv/vmbus_drv.c
>>>> index 172ceae69abb..4bc02aea2098 100644
>>>> --- a/drivers/hv/vmbus_drv.c
>>>> +++ b/drivers/hv/vmbus_drv.c
>>>> @@ -31,6 +31,7 @@
>>>>    #include <linux/kdebug.h>
>>>>    #include <linux/efi.h>
>>>>    #include <linux/random.h>
>>>> +#include <linux/kernel.h>
>>>
>>> Unfortunately, adding the #include doesn't solve the problem.  The error occurs when
>>> CONFIG_HYPERV=m, because panic_on_oops is not exported.  I haven't thought it
>>> through, but hopefully there's a solution where panic_on_oops can be tested in
>>> hyperv_report_panic() or some other Hyper-V specific function that's never in a
>>> module, so that we don't need to export panic_on_oops.
>>
>> Yes, I don't consider modules case. I think we may introduce a check
>> function of panic_on_oops in the mshyperv.c and expose it to module.
>>
> 
> Why expose something new? You can just test panic_on_oops in
> hyperv_report_panic and bail if it is false, right?
> 
> Something like the following (not compiled) diff:
> 
> diff --git a/arch/x86/hyperv/hv_init.c b/arch/x86/hyperv/hv_init.c
> index b0da5320bcff..0dc229a9142c 100644
> --- a/arch/x86/hyperv/hv_init.c
> +++ b/arch/x86/hyperv/hv_init.c
> @@ -424,6 +424,9 @@ void hyperv_report_panic(struct pt_regs *regs, long err)
>          static bool panic_reported;
>          u64 guest_id;
> 
> +       if (!panic_on_oops)
> +               return;
> +
>          /*
>           * We prefer to report panic on 'die' chain as we have proper
>           * registers to report, but if we miss it (e.g. on BUG()) we need
> 
> I haven't checked all the error reporting paths and don't know if this
> works or not though.

Hi Wei:
     hyperv_report_panic() is also called in panic() which may not be
triggered by die(). In these cases, we still need to report crash
register data to host even when panic_on_oops is unset.
     Another approach is to add new parameter "in_die" for
hyperv_report_panic() and just check panic_on_oops when the function is
called in die notifier callback and "in_die" is set to true.