Re: [Qemu-devel] [PATCH] vfio: fix duplicate function call

[Cao jin <caoj.fnst@cn.fujitsu.com>]

On 10/17/2016 11:01 PM, Alex Williamson wrote:
> On Mon, 17 Oct 2016 16:57:08 +0800
> Cao jin <caoj.fnst@cn.fujitsu.com> wrote:
>
>> Hi,
>>
>> On 10/14/2016 11:50 PM, Alex Williamson wrote:
>>> On Fri, 14 Oct 2016 19:16:59 +0800
>>> Cao jin <caoj.fnst@cn.fujitsu.com> wrote:
>>>
>>>> When vfio device is reset(encounter FLR, or bus reset), if need to do
>>>> bus reset(vfio_pci_hot_reset_one is called), vfio_pci_pre_reset &
>>>> vfio_pci_post_reset will be called twice.
>>>>
>>>> Signed-off-by: Cao jin <caoj.fnst@cn.fujitsu.com>
>>>> ---
>>>> Also has a little question on vfio_pci_reset. it will be called when encounter
>>>> bus reset, or FLR. The reset method's priority in this function now is:
>>>>
>>>>       1. If has "device specific reset function", then do it
>>>>       2. If has FLR, then do it.
>>>>       3. If it can do bus reset(only 1 affected device), then do it
>>>>       4. If has pm_reset, then do it
>>>>
>>>> The question is: why pm reset has low priority than bus reset(if it does
>>>> can do a bus reset)? why bus reset is not the last choice? In PCI driver
>>>> of kernel, pls see __pci_dev_reset, we can see, if device support pm reset,
>>>> it won't do bus reset.
>>>
>>> The PCI spec doesn't really define what sort of reset is done with a PM
>>> reset.  My thinking was that if a device advertises an FLR capability
>>> then the hardware has made a concerted effort to have a per function
>>> reset mechanism available.  NoSoftRst- is not terribly common and it's
>>> not entirely clear to me that the hardware has made a conscious effort
>>> to provide this for the purposes of per function reset mechanism.
>>> Therefore I've opt'd to prioritize a bus reset over a PM reset.
>>>
>>
>> I still have a question about vfio_pci_reset. I checked commit message
>> in f16f39c3, if I understand right, couldn't we put
>>
>>       /* See if we can do our own bus reset */
>>       if (!vfio_pci_hot_reset_one(vdev)) {
>>           goto post_reset;
>>       }
>>
>> in the 1st priority? Because if there is 1 affected device, then it will
>> do bus reset which is the best reset we can do; if there are more than 1
>> affected devices, after this patch, vfio_pci_hot_reset_one will do
>> nothing, and then try other reset methods.
>
> It's possible, yes, but that disregards that the hardware has gone to
> the trouble to implement a proper function level reset.  As I
> explained, I de-prioritize PM reset, specifically because I'm not sure
> if hardware designers are necessarily intending it for the purpose of a
> device reset.  For FLR this is the entire purpose of the interface.  We
> also have a fair bit of experience with the current priority scheme and
> I would not take it lightly to change without some compelling evidence
> to prove that a new priority scheme is better than the existing.  There
> do also exist devices which do not behave properly with a secondary bus
> reset, see drivers/pci/quirks.c:quirk_no_bus_reset() in the kernel
> tree.  It's possible more devices like this exist, but we don't see
> them because they implement FLR.  A bus reset may result in a more
> complete device reset, but it's also more disruptive to the system.
> Thanks,
>
> Alex
>

I see. Thanks Alex, I think these are valuable info to me, although 
maybe I still need more time in the future to understand these totally.

-- 
Yours Sincerely,

Cao jin