linux-kernel - RE: [PATCH v2] pci: fix device presence detection for VFs

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Date:   Thu, 17 Nov 2022 05:36:48 +0000
From:   Parav Pandit <parav@...dia.com>
To:     Lukas Wunner <lukas@...ner.de>,
        "Michael S. Tsirkin" <mst@...hat.com>
CC:     Bjorn Helgaas <helgaas@...nel.org>,
        Wei Gong <gongwei833x@...il.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        "linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>
Subject: RE: [PATCH v2] pci: fix device presence detection for VFs


> From: Lukas Wunner <lukas@...ner.de>
> Sent: Wednesday, November 16, 2022 6:16 AM
> 
> [cc += Parav Pandit, author of 43bb40c5b926]
> 
> On Sun, Nov 13, 2022 at 03:46:06AM -0500, Michael S. Tsirkin wrote:
> > On Fri, Nov 11, 2022 at 05:42:19PM -0600, Bjorn Helgaas wrote:
> > > On Thu, Nov 10, 2022 at 03:15:55PM -0500, Michael S. Tsirkin wrote:
> > > > On Thu, Nov 10, 2022 at 01:35:47PM -0600, Bjorn Helgaas wrote:
> > > > > Prior to this change pci_device_is_present(VF) returned "false"
> > > > > (because the VF Vendor ID is 0xffff); after the change it will
> > > > > return "true" (because it will look at the PF Vendor ID instead).
> > > > >
> > > > > Previously virtio_pci_remove() called virtio_break_device().  I
> > > > > guess that meant the virtio I/O operation will never be completed?
> > > > >
> > > > > But if we don't call virtio_break_device(), the virtio I/O
> > > > > operation
> > > > > *will* be completed?
> >
> > Just making sure - pci_device_is_present *is* the suggested way to
> > distinguish between graceful and surprise removal, isn't it?
> 
> No, it's not.  Instead of !pci_device_is_present() you really want to call
> pci_dev_is_disconnected() instead.
> 
> While the fix Bjorn applied for v6.2 may solve the issue and may make sense
> on it's own, it's not the solution you're looking for.  You want to swap the
> call to !pci_device_is_present() with pci_dev_is_disconnected(), move
> pci_dev_is_disconnected() from drivers/pci/pci.h to include/linux/pci.h and
> add a Fixes tag referencing 43bb40c5b926.
> 
> If you don't want to move pci_dev_is_disconnected(), you can alternatively
> check for "pdev->error_state == pci_channel_io_perm_failure" or call
> pci_channel_offline().  The latter will also return true though on transient
> inaccessibility of the device (e.g. if it's being reset).
> 
pci_device_is_present() is calling pci_dev_is_disconnected().
pci_dev_is_disconnected() avoids reading the vendor id.
So pci_dev_is_disconnected() looks less strong check.
I see that it can return a valid value on recoverable error case.

In that case, is pci_channel_offline() a more precise way to check that covers transient and permanent error?

And if that is the right check, we need to fix all the callers, mainly widely used nvme driver [1].

[1] https://elixir.bootlin.com/linux/v6.1-rc5/source/drivers/nvme/host/pci.c#L3228

Also, we need to add API documentation on when to use this API in context of hotplug, so that all related drivers can consistently use single API.

> The theory of operation is as follows:  The PCI layer does indeed know
> whether the device was surprise removed or gracefully removed and that
> information is passed in the "presence" flag to pciehp_unconfigure_device()
> (in drivers/pci/hotplug/pciehp_pci.c).  That function does the following:
> 
> 	if (!presence)
> 		pci_walk_bus(parent, pci_dev_set_disconnected, NULL);
> 
> In other words, pdev->error_state is set to pci_channel_io_perm_failure on
> the entire hierarchy below the hotplug port.  And pci_dev_is_disconnected()
> simply checks whether that's the device's error_state.
> 
> pci_dev_is_disconnected() makes sense if you definitely know the device is
> gone and want to skip certain steps or delays on device teardown.
> However be aware that the device may be hot-removed after graceful
> removal was initiated.  In such a situation, pci_dev_is_disconnected() may
> return false and you'll try to access the device as normal, even though it was
> yanked from the slot after the pci_dev_is_disconnected() call was
> performed.  Ideally you should be able to cope with such scenarios as well.
> 
> For some more background info, refer to this LWN article (scroll down to the
> "Surprise removal" section):
> https://lwn.net/Articles/767885/
> 
> Thanks,
> 
> Lukas