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

[Alex Williamson <alex.williamson@redhat.com>]

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