Re: [PATCH v3 1/3] msix/hmp: add hmp interface to dump MSI-X info

[Dongli Zhang]

Hi Markus,

On 7/14/21 2:42 AM, Markus Armbruster wrote:
> Dongli Zhang <dongli.zhang@oracle.com> writes:
> 
>> Hi Markus,
>>
>> On 7/13/21 10:46 PM, Markus Armbruster wrote:
>>> Dongli Zhang <dongli.zhang@oracle.com> writes:
>>>
>>>> This patch is to add the HMP interface to dump MSI-X table and PBA, in
>>>> order to help diagnose the loss of IRQ issue in VM (e.g., if an MSI-X
>>>> vector is erroneously masked permanently). Here is the example with
>>>> vhost-scsi:
>>>>
>>>> (qemu) info msix /machine/peripheral/vscsi0
>>>> Msg L.Addr Msg U.Addr Msg Data   Vect Ctrl
>>>> 0xfee00000 0x00000000 0x00004041 0x00000000
>>>> 0xfee00000 0x00000000 0x00004051 0x00000000
>>>> 0xfee00000 0x00000000 0x00004061 0x00000000
>>>> 0xfee00000 0x00000000 0x00004071 0x00000000
>>>> 0xfee01000 0x00000000 0x000040b1 0x00000000
>>>> 0xfee02000 0x00000000 0x000040c1 0x00000000
>>>> 0xfee03000 0x00000000 0x000040d1 0x00000000
>>>>
>>>> MSI-X PBA
>>>> 0 0 0 0 0 0 0
>>>>
>>>> Since the number of MSI-X entries is not determined and might be very
>>>> large, it is sometimes inappropriate to dump via QMP.
>>>
>>> Why?  What makes HMP different?
>>
>> Here are two reasons.
>>
>> 1. The size of MSI-X table is nondeterministic and might be very large, e.g.,
>> the PCI_MSIX_FLAGS_QSIZE is 0x07FF. The "info tlb" (which is a table and 
>> similar
>> to MSI-X) and "info lapic" also only support hmp.
>>
>> 2. The [PATCH 3/3] of this patchset support device specific data, the
>> definitional of which varies depending on each device type (so far only
>> virtio-pci supports the interface).
>>
>> Thank you very much!
>>
>> Dongli Zhang
>>
>>>
>>>> Therefore, this patch dumps MSI-X information only via HMP, which is
>>>> similar to the implementation of hmp_info_mem().
> 
> I think you're mixing up valid and invalid arguments :)  Let me try to
> pick them apart.
> 
> 1a. "Command output may be too large for QMP, therefore provide only the
> HMP command" makes no sense.
> 
> Both QMP and HMP are not meant for bulk data transfer.  They are control
> plane, not data plane.  To illustrate what that means, consider a backup
> feature.  The commands to control the backup task are QMP (and HMP, if
> desired), but the actual data transfer uses some other channel, so it
> doesn't clog the QMP pipes.
> 
> If the data is too bulky for QMP, then it's too bulky for HMP, too.
> 
> 1b. "info tlb" and "info lapic" are HMP only because they are debugging
> commands for humans.  Debugging is not necessarily done by humans only.
> Sometimes, humans use programs to help them debug, and these programs
> would be better off with QMP commands.  For the HMP-only debugging
> commands, we decided providing for (largely hypothetical) debugging
> scripts wasn't worthwhile.  A similar argument could probably be made
> for "info msix".
> 
> 2. "Output is in part specific to Devices, therefore provide only the
> HMP command" is also iffy.  Yes, hacking up a bunch of monitor_printf()s
> is probably easier than specifying a QAPI schema.   "It's work" is no
> excuse.  "It's more work than it's worth" can be one.  But then we're
> back at argument 1b.

Thank you very much for the explanation!

> 
> Your commit message's first sentence suggests this is indeed just for
> debugging.  If this is the case, consider replacing
> 
>     Since the number of MSI-X entries is not determined and might be very
>     large, it is sometimes inappropriate to dump via QMP.
> 
>     Therefore, this patch dumps MSI-X information only via HMP, which is
>     similar to the implementation of hmp_info_mem().
> 
> by
> 
>     Since this is just for debugging by humans, provide the command only
>     in HMP, not in QMP.
> 

Yes, this is for debugging purpose. The primary objective is to confirm a
specific vector is not erroneously masked permanently, when a specific device
queue is stuck.

I will replace commit message and send v4.

Thank you very much!

Dongli Zhang