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Date:   Wed, 17 Mar 2021 16:53:09 +0530
From:   Amey Narkhede <ameynarkhede03@gmail.com>
To:     Leon Romanovsky <leon@kernel.org>
Cc:     alex.williamson@redhat.com, raphael.norwitz@nutanix.com,
        linux-pci@vger.kernel.org, bhelgaas@google.com,
        linux-kernel@vger.kernel.org, alay.shah@nutanix.com,
        suresh.gumpula@nutanix.com, shyam.rajendran@nutanix.com,
        felipe@nutanix.com
Subject: Re: [PATCH 4/4] PCI/sysfs: Allow userspace to query and set device
 reset mechanism
Message-ID: <20210317112309.nborigwfd26px2mj@archlinux>
References: <20210315134323.llz2o7yhezwgealp@archlinux>
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 <20210315183226.GA14801@raphael-debian-dev>
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On 21/03/17 01:02PM, Leon Romanovsky wrote:
> On Wed, Mar 17, 2021 at 03:54:47PM +0530, Amey Narkhede wrote:
> > On 21/03/17 06:20AM, Leon Romanovsky wrote:
> > > On Mon, Mar 15, 2021 at 06:32:32PM +0000, Raphael Norwitz wrote:
> > > > On Mon, Mar 15, 2021 at 10:29:50AM -0600, Alex Williamson wrote:
> > > > > On Mon, 15 Mar 2021 21:03:41 +0530
> > > > > Amey Narkhede <ameynarkhede03@gmail.com> wrote:
> > > > >
> > > > > > On 21/03/15 05:07PM, Leon Romanovsky wrote:
> > > > > > > On Mon, Mar 15, 2021 at 08:34:09AM -0600, Alex Williamson wrote:
> > > > > > > > On Mon, 15 Mar 2021 14:52:26 +0100
> > > > > > > > Pali Roh?r <pali@kernel.org> wrote:
> > > > > > > >
> > > > > > > > > On Monday 15 March 2021 19:13:23 Amey Narkhede wrote:
> > > > > > > > > > slot reset (pci_dev_reset_slot_function) and secondary bus
> > > > > > > > > > reset(pci_parent_bus_reset) which I think are hot reset and
> > > > > > > > > > warm reset respectively.
> > > > > > > > >
> > > > > > > > > No. PCI secondary bus reset = PCIe Hot Reset. Slot reset is just another
> > > > > > > > > type of reset, which is currently implemented only for PCIe hot plug
> > > > > > > > > bridges and for PowerPC PowerNV platform and it just call PCI secondary
> > > > > > > > > bus reset with some other hook. PCIe Warm Reset does not have API in
> > > > > > > > > kernel and therefore drivers do not export this type of reset via any
> > > > > > > > > kernel function (yet).
> > > > > > > >
> > > > > > > > Warm reset is beyond the scope of this series, but could be implemented
> > > > > > > > in a compatible way to fit within the pci_reset_fn_methods[] array
> > > > > > > > defined here.  Note that with this series the resets available through
> > > > > > > > pci_reset_function() and the per device reset attribute is sysfs remain
> > > > > > > > exactly the same as they are currently.  The bus and slot reset
> > > > > > > > methods used here are limited to devices where only a single function is
> > > > > > > > affected by the reset, therefore it is not like the patch you proposed
> > > > > > > > which performed a reset irrespective of the downstream devices.  This
> > > > > > > > series only enables selection of the existing methods.  Thanks,
> > > > > > >
> > > > > > > Alex,
> > > > > > >
> > > > > > > I asked the patch author here [1], but didn't get any response, maybe
> > > > > > > you can answer me. What is the use case scenario for this functionality?
> > > > > > >
> > > > > > > Thanks
> > > > > > >
> > > > > > > [1] https://lore.kernel.org/lkml/YE389lAqjJSeTolM@unreal/
> > > > > > >
> > > > > > Sorry for not responding immediately. There were some buggy wifi cards
> > > > > > which needed FLR explicitly not sure if that behavior is fixed in
> > > > > > drivers. Also there is use a case at Nutanix but the engineer who
> > > > > > is involved is on PTO that is why I did not respond immediately as
> > > > > > I don't know the details yet.
> > > > >
> > > > > And more generally, devices continue to have reset issues and we
> > > > > impose a fixed priority in our ordering.  We can and probably should
> > > > > continue to quirk devices when we find broken resets so that we have
> > > > > the best default behavior, but it's currently not easy for an end user
> > > > > to experiment, ie. this reset works, that one doesn't.  We might also
> > > > > have platform issues where a given reset works better on a certain
> > > > > platform.  Exposing a way to test these things might lead to better
> > > > > quirks.  In the case I think Pali was looking for, they wanted a
> > > > > mechanism to force a bus reset, if this was in reference to a single
> > > > > function device, this could be accomplished by setting a priority for
> > > > > that mechanism, which would translate to not only the sysfs reset
> > > > > attribute, but also the reset mechanism used by vfio-pci.  Thanks,
> > > > >
> > > > > Alex
> > > > >
> > > >
> > > > To confirm from our end - we have seen many such instances where default
> > > > reset methods have not worked well on our platform. Debugging these
> > > > issues is painful in practice, and this interface would make it far
> > > > easier.
> > > >
> > > > Having an interface like this would also help us better communicate the
> > > > issues we find with upstream. Allowing others to more easily test our
> > > > (or other entities') findings should give better visibility into
> > > > which issues apply to the device in general and which are platform
> > > > specific. In disambiguating the former from the latter, we should be
> > > > able to better quirk devices for everyone, and in the latter cases, this
> > > > interface allows for a safer and more elegant solution than any of the
> > > > current alternatives.
> > >
> > > So to summarize, we are talking about test and debug interface to
> > > overcome HW bugs, am I right?
> > >
> > > My personal experience shows that once the easy workaround exists
> > > (and write to generally available sysfs is very simple), the vendors
> > > and users desire for proper fix decreases drastically. IMHO, we will
> > > see increase of copy/paste in SO and blog posts, but reduce in quirks.
> > >
> > > My 2-cents.
> > >
> > I agree with your point but at least it gives the userspace ability
> > to use broken device until bug is fixed in upstream.
>
> As I said, I don't expect many fixes once "userspace" will be able to
> use cheap workaround. There is no incentive to fix it.
>
> > This is also applicable for obscure devices without upstream
> > drivers for example custom FPGA based devices.
>
> This is not relevant to upstream kernel. Those vendors ship everything
> custom, they don't need upstream, we don't need them :)
>
By custom I meant hobbyists who could tinker with their custom FPGA.

> > Another main application which I forgot to mention is virtualization
> > where vmm wants to reset the device when the guest is reset,
> > to emulate machine reboot as closely as possible.
>
> It can work in very narrow case, because reset will cause to device
> reprobe and most likely the driver will be different from the one that
> started reset. I can imagine that net devices will lose their state and
> config after such reset too.
>
Not sure if I got that 100% right. The pci_reset_function() function
saves and restores device state over the reset.

> IMHO, it will be saner for everyone if virtualization don't try such resets.
>
> Thanks
>
The exists reset sysfs attribute was added for exactly this case
though.

Thanks,
Amey