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SubjectRe: Qemu Guest kernel 4.20-rc1 PCIe hotplug issue
On Wed, Nov 14, 2018 at 02:30:14PM +0100, Lukas Wunner wrote:
> On Wed, Nov 14, 2018 at 11:52:25AM +0200, mika.westerberg@linux.intel.com wrote:
> > On Tue, Nov 13, 2018 at 03:57:47PM +0000, Shameerali Kolothum Thodi wrote:
> > > > The smb_mb() thing is not that clear (at least to me) because it is used
> > > > in two places in the driver and both seem to be making write to
> > > > ctrl->cmd_busy visible to other CPUs but I don't see where we deal with
> > > > the read part.
> > > >
> > > > I may be missing something, though.
> > >
> > > I think the read part is in wait_event_timeout() which evaluates the
> > > condition. The wake_up is called from the pciehp_isr(). Since the flag
> > > is being updated in both process level and interrupt handler context,
> > > smp_mb() is used. I think the same now applies to ctrl->slot_ctrl now
> > > as this being used in process context and interrupt context as well.
> >
> > Right, but that would require to use another read/general barrier in the
> > pciehp_isr() before we read the variable in case interrupt happens
> > immediately on another CPU (at least that's my understanding).
>
> In pcie_do_write_cmd(), please just move the
>
> ctrl->slot_ctrl = slot_ctrl;
>
> above the call to pcie_capability_write_word().
>
> AFAICS an explicit memory barrier isn't needed here because of the call to
> pcie_capability_write_word(), which "will [ordinarily] be guaranteed to be
> fully ordered and uncombined" (Documentation/memory-barriers.txt, section
> "KERNEL I/O BARRIER EFFECTS").
>
> The memory barrier in pciehp_isr() is also bogus because the following
> wake_up() implies a memory barrier if a task was woken. (And if none
> was woken, who cares.)
>
>
> > Since I'm
> > not too comfortable with all these barriers to be honest I would prefer
> > reading the slot control register directly in pciehp_isr() :-)
>
> That is an approach I'd strongly object to: While pciehp itself only
> signals very few interrupts (making an additional mmio read appear to
> be negligible), it may share its interrupt with other devices. On my
> MacBookPro9,1, a hotplug port of the Thunderbolt controller shares
> its interrupt line with the Wifi card and SD card reader, and those
> may signal a huge number of interrupts. On such a machine an additional
> mmio read per interrupt becomes a problem.

OK.

I just sent a patch moving ctrl->slot_ctrl assignment to happen before
pcie_capability_write_word().

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