Re: [Xen-devel] [PATCH 1/2] xen, libxc: init msix addr/data with value from qemu via hypercall

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.05.13 at 09:39, Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> wrote:

> On 2013-05-10 14:37, Jan Beulich wrote:
>>>>> On 10.05.13 at 04:49, Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> wrote:
>>> On 2013-05-10 03:05, Jan Beulich wrote:
>>>>>>> Zhenzhong Duan <zhenzhong.duan@xxxxxxxxxx> 05/09/13 5:02 AM >>>
>>>>> On 2013/5/8 20:03, Jan Beulich wrote:
>>>>>> But of course I still don't really understand why all of the sudden
>>>>>> this needs to be passed in rather than being under the full control
>>>>>> of the hypervisor at all times. Perhaps this is related to me not
>>>>>> understanding why the kernel would read these values at all:
>>>>>> There's no other place in the kernel where the message would
>>>>>> be read before first getting written (in fact, apart from the
>>>>>> use of __read_msi_msg() by the Xen code, there's only one
>>>>>> other user under arch/powerpc/, and there - according to the
>>>>>> accompanying comment - this is just to save away the data for
>>>>>> later use during resume).
>>>>> There is a bug if msi_ad is not passed in.
>>>>>
>>>>> when driver first load,
>>>>>
>>>>> kernel.__read_msi_msg()
>>>>> (got all zero)
>>>> But you don't even comment on the apparently bogus use of the function 
>>>> here.
>>> This pattern is used only when hvm_pirq is enabled. kernel need to check
>>> XEN_PIRQ_MSI_DATA.
>>> It's not a issue if data is 0 at first driver load, kernel will call
>>> __write_msi_msg with pirq and  XEN_PIRQ_MSI_DATA set.
>> But this doesn't make the use of __read_msi_msg() less bogus. It's
>> not clear on what basis this mechanism got invented in the first
>> place.
> It's there since hvm_irq introduced. But it works indeed.

But that doesn't in any way mean the concept is sound.

>>>>> kernel.__write_msi_msg(pirq)
>>>>> (ioreq passed to qemu as no msixtbl_entry established yet)
>>>>> qemu.pt_msi_update_one()
>>>>> xc_domain_update_msi_irq()
>>>>> (msixtbl_entry dynamicly allocated with msi_ad all zero)
>>>>>
>>>>> then driver unload,
>>>>> ...
>>>>> driver load again,
>>>>>
>>>>> kernel.__read_msi_msg()
>>>>> (got all zero from xen as accelerated entry just established with all 
>>>>> zero)
>>>> If all zeroes get returned, why would the flow here be different then 
>>>> above?
>>> Because pirq and related mapping and binding are not freed between
>>> driver load-unload-load. They are freed when device detach.
>>> We should try to use the last pirq.
>> But then you need to solve the problem generically, i.e. not just
>> for the driver reload case, but also for e.g. the kexec one (where
>> __read_msi_msg() returning other than all zeros wouldn't help you
>> as xen_irq_from_pirq() would then return -1, and you'd be back to
>> the same problem.
> No, not only kexec ones, it's driver unload that makes xen_irq_from_pirq 
> return -1. So there is also a bug in kernel side.
> I have sent a patch about kernel. I think you miss it.
> http://www.gossamer-threads.com/lists/xen/devel/281498 
>> IOW I think the prior IRQ needs to be freed
>> anyway rather than an attempt be made to reuse it.
> I have ever thought about this idea, but when to free the pirq is a problem.
> When driver unload? qemu has no idea of if driver unloaded.

But the kernel does, and hence could deal with this. As much as
the setup is being done when the driver gets loaded, cleanup
should be done when the driver gets unloaded. _If_ there
already is such an odd protocol between kernel and qemu, then
if that can't be dropped, it surely can be leveraged to also deal
with the cleanup side of things? No need to fiddle with the
hypervisor interfaces for something that it's not supposed to
know about anyway.

> When msix entry masked? kernel mask and unmask msix entry 
> intermittently, especially when irqbalance enabled.
> 
> So based on above, I think it's better to reuse same pirq, only free it 
> when device detached.

I continue to disagree. Also from a theoretical perspective - if you
have a lot of devices that no driver is loaded for, you'd keep a lot
of IRQs allocated without any need.

Jan

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