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From: Kuppuswamy Sathyanarayanan  <sathyanarayanan.kuppuswamy@linux.intel.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
	ashok.raj@intel.com, keith.busch@intel.com,
	Joerg Roedel <joro@8bytes.org>,
	David Woodhouse <dwmw2@infradead.org>,
	iommu@lists.linux-foundation.org
Subject: Re: [PATCH v5 4/7] PCI/ATS: Add PRI support for PCIe VF devices
Date: Mon, 19 Aug 2019 15:53:31 -0700
Message-ID: <20190819225331.GB28404@skuppusw-desk.amr.corp.intel.com> (raw)
In-Reply-To: <20190819141500.GQ253360@google.com>

On Mon, Aug 19, 2019 at 09:15:00AM -0500, Bjorn Helgaas wrote:
> On Thu, Aug 15, 2019 at 03:39:03PM -0700, Kuppuswamy Sathyanarayanan wrote:
> > On 8/15/19 3:20 PM, Bjorn Helgaas wrote:
> > > [+cc Joerg, David, iommu list: because IOMMU drivers are the only
> > > callers of pci_enable_pri() and pci_enable_pasid()]
> > > 
> > > On Thu, Aug 01, 2019 at 05:06:01PM -0700, sathyanarayanan.kuppuswamy@linux.intel.com wrote:
> > > > From: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>
> > > > 
> > > > When IOMMU tries to enable Page Request Interface (PRI) for VF device
> > > > in iommu_enable_dev_iotlb(), it always fails because PRI support for
> > > > PCIe VF device is currently broken. Current implementation expects
> > > > the given PCIe device (PF & VF) to implement PRI capability before
> > > > enabling the PRI support. But this assumption is incorrect. As per PCIe
> > > > spec r4.0, sec 9.3.7.11, all VFs associated with PF can only use the
> > > > PRI of the PF and not implement it. Hence we need to create exception
> > > > for handling the PRI support for PCIe VF device.
> > > > 
> > > > Also, since PRI is a shared resource between PF/VF, following rules
> > > > should apply.
> > > > 
> > > > 1. Use proper locking before accessing/modifying PF resources in VF
> > > >     PRI enable/disable call.
> > > > 2. Use reference count logic to track the usage of PRI resource.
> > > > 3. Disable PRI only if the PRI reference count (pri_ref_cnt) is zero.
> 
> > > Wait, why do we need this at all?  I agree the spec says VFs may not
> > > implement PRI or PASID capabilities and that VFs use the PRI and
> > > PASID of the PF.
> > > 
> > > But why do we need to support pci_enable_pri() and pci_enable_pasid()
> > > for VFs?  There's nothing interesting we can *do* in the VF, and
> > > passing it off to the PF adds all this locking mess.  For VFs, can we
> > > just make them do nothing or return -EINVAL?  What functionality would
> > > we be missing if we did that?
> > 
> > Currently PRI/PASID capabilities are not enabled by default. IOMMU can
> > enable PRI/PASID for VF first (and not enable it for PF). In this case,
> > doing nothing for VF device will break the functionality.
> 
> What is the path where we can enable PRI/PASID for VF but not for the
> PF?  The call chains leading to pci_enable_pri() go through the
> iommu_ops.add_device interface, which makes me think this is part of
> the device enumeration done by the PCI core, and in that case I would
> think this it should be done for the PF before VFs.  But maybe this
> path isn't exercised until a driver does a DMA map or something
> similar?
AFAIK, this path will only get exercised when the device does DMA and
hence there is no specific order in which PRI/PASID is enabled in PF/VF.
In fact, my v2 version of this patch set had a check to ensure PF
PRI/PASID enable is happened before VF attempts PRI/PASID
enable/disable. But I had to remove it in later version of this series
due to failure case reported by one the tester of this code. 
> 
> Bjorn

-- 
-- 
Sathyanarayanan Kuppuswamy
Linux kernel developer

  reply index

Thread overview: 36+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-08-02  0:05 [PATCH v5 0/7] Fix PF/VF dependency issue sathyanarayanan.kuppuswamy
2019-08-02  0:05 ` [PATCH v5 1/7] PCI/ATS: Fix pci_prg_resp_pasid_required() dependency issues sathyanarayanan.kuppuswamy
2019-08-12 20:04   ` Bjorn Helgaas
2019-08-12 20:20     ` sathyanarayanan kuppuswamy
2019-08-13  3:51       ` Bjorn Helgaas
2019-08-16 18:06         ` Kuppuswamy Sathyanarayanan
2019-08-02  0:05 ` [PATCH v5 2/7] PCI/ATS: Initialize PRI in pci_ats_init() sathyanarayanan.kuppuswamy
2019-08-12 20:04   ` Bjorn Helgaas
2019-08-12 21:35     ` sathyanarayanan kuppuswamy
2019-08-13  4:10       ` Bjorn Helgaas
2019-08-15  4:46   ` Bjorn Helgaas
2019-08-15 17:30     ` Kuppuswamy Sathyanarayanan
2019-08-16 17:31       ` Bjorn Helgaas
2019-08-02  0:06 ` [PATCH v5 3/7] PCI/ATS: Initialize PASID " sathyanarayanan.kuppuswamy
2019-08-12 20:04   ` Bjorn Helgaas
2019-08-15  4:48   ` Bjorn Helgaas
2019-08-15  4:56   ` Bjorn Helgaas
2019-08-15 17:31     ` Kuppuswamy Sathyanarayanan
2019-08-02  0:06 ` [PATCH v5 4/7] PCI/ATS: Add PRI support for PCIe VF devices sathyanarayanan.kuppuswamy
2019-08-12 20:04   ` Bjorn Helgaas
2019-08-12 21:40     ` sathyanarayanan kuppuswamy
2019-08-13  4:16   ` Bjorn Helgaas
2019-08-15 22:20   ` Bjorn Helgaas
2019-08-15 22:39     ` Kuppuswamy Sathyanarayanan
2019-08-19 14:15       ` Bjorn Helgaas
2019-08-19 22:53         ` Kuppuswamy Sathyanarayanan [this message]
2019-08-19 23:19           ` Bjorn Helgaas
2019-08-28 18:21             ` Kuppuswamy Sathyanarayanan
2019-08-28 18:57               ` Bjorn Helgaas
2019-08-02  0:06 ` [PATCH v5 5/7] PCI/ATS: Add PASID " sathyanarayanan.kuppuswamy
2019-08-12 20:05   ` Bjorn Helgaas
2019-08-13 22:19     ` Kuppuswamy Sathyanarayanan
2019-08-15  5:04       ` Bjorn Helgaas
2019-08-16  1:21         ` Kuppuswamy Sathyanarayanan
2019-08-02  0:06 ` [PATCH v5 6/7] PCI/ATS: Disable PF/VF ATS service independently sathyanarayanan.kuppuswamy
2019-08-02  0:06 ` [PATCH v5 7/7] PCI: Skip Enhanced Allocation (EA) initialization for VF device sathyanarayanan.kuppuswamy

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