On Mon, 17 Oct 2016 16:57:08 +0800
Cao jin <caoj.f...@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.f...@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.f...@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
