completion timeouts with pin-based interrupts in QEMU hw/nvme

[Peter Maydell]

On Tue, 17 Jan 2023 at 16:10, Guenter Roeck <linux at roeck-us.net> wrote:
>
> On Mon, Jan 16, 2023 at 09:58:13PM -0700, Keith Busch wrote:
> > On Mon, Jan 16, 2023 at 10:14:07PM +0100, Klaus Jensen wrote:
> > > I noticed that the Linux driver does not use the INTMS/INTMC registers
> > > to mask interrupts on the controller while processing CQEs. While not
> > > required by the spec, it is *recommended* in setups not using MSI-X to
> > > reduce the risk of spurious and/or missed interrupts.
> >
> > That's assuming completions are deferred to a bottom half. We don't do
> > that by default in Linux nvme, though you can ask the driver to do that
> > if you want.
> >
> > > With the patch below, running 100 boot iterations, no timeouts were
> > > observed on QEMU emulated riscv64 or mips64.
> > >
> > > No changes are required in the QEMU hw/nvme interrupt logic.
> >
> > Yeah, I can see why: it forces the irq line to deassert then assert,
> > just like we had forced to happen within the device side patches. Still,
> > none of that is supposed to be necessary, but this idea of using these
> > registers is probably fine.
>
> There is still no answer why this would be necessary in the first place,
> on either side. In my opinion, unless someone can confirm that the problem
> is seen with real hardware, we should assume that it happens on the qemu
> side and address it there.

Sure, but that means identifying what the divergence
between QEMU's implementation and the hardware is first. I don't
want a fudged fix in QEMU's code any more than you want one in
the kernel's driver code :-)

thanks
-- PMM