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

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Message-ID: <20221109020614-mutt-send-email-mst@kernel.org>
Date:   Wed, 9 Nov 2022 02:10:30 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Bjorn Helgaas <helgaas@...nel.org>
Cc:     Wei Gong <gongwei833x@...il.com>, linux-kernel@...r.kernel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>, linux-pci@...r.kernel.org
Subject: Re: [PATCH v2] pci: fix device presence detection for VFs

On Tue, Nov 08, 2022 at 11:12:34PM -0600, Bjorn Helgaas wrote:
> On Wed, Nov 09, 2022 at 04:36:17AM +0000, Wei Gong wrote:
> > O Tue, Nov 08, 2022 at 01:02:35PM -0500, Michael S. Tsirkin wrote:
> > > On Tue, Nov 08, 2022 at 11:58:53AM -0600, Bjorn Helgaas wrote:
> > > > On Tue, Nov 08, 2022 at 10:19:07AM -0500, Michael S. Tsirkin wrote:
> > > > > On Tue, Nov 08, 2022 at 09:02:28AM -0600, Bjorn Helgaas wrote:
> > > > > > On Tue, Nov 08, 2022 at 08:53:00AM -0600, Bjorn Helgaas wrote:
> > > > > > > On Wed, Oct 26, 2022 at 02:11:21AM -0400, Michael S. Tsirkin wrote:
> > > > > > > > virtio uses the same driver for VFs and PFs.
> > > > > > > > Accordingly, pci_device_is_present is used to detect
> > > > > > > > device presence. This function isn't currently working
> > > > > > > > properly for VFs since it attempts reading device and
> > > > > > > > vendor ID.
> > > > > > > 
> > > > > > > > As VFs are present if and only if PF is present,
> > > > > > > > just return the value for that device.
> > > > > > > 
> > > > > > > VFs are only present when the PF is present *and* the PF
> > > > > > > has VF Enable set.  Do you care about the possibility that
> > > > > > > VF Enable has been cleared?
> > > > 
> > > > I think you missed this question.
> > > 
> > > I was hoping Wei will answer that, I don't have the hardware.
> > 
> > In my case I don't care that VF Enable has been cleared.
> 
> OK, let me rephrase that :)
> 
> I think pci_device_is_present(VF) should return "false" if the PF is
> present but VFs are disabled.
> 
> If you think it should return "true" when the PF is present and VFs
> are disabled, we should explain why.
> 
> We would also need to fix the commit log, because "VFs are present if
> and only if PF is present" is not actually true.  "VFs are present
> only if PF is present" is true, but "VFs are present if PF is present"
> is not.
> 
> Bjorn

Bjorn, I don't really understand the question.

How does one get a vf pointer without enabling sriov?
They are only created by sriov_add_vfs after calling
pcibios_sriov_enable.



-- 
MST