Re: [PATCH v7 06/11] PCI: portdrv: Set driver_managed_dma

From: Lu Baolu <baolu.lu@linux.intel.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: baolu.lu@linux.intel.com,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Joerg Roedel <joro@8bytes.org>,
	Alex Williamson <alex.williamson@redhat.com>,
	Bjorn Helgaas <bhelgaas@google.com>,
	Jason Gunthorpe <jgg@nvidia.com>,
	Christoph Hellwig <hch@infradead.org>,
	Kevin Tian <kevin.tian@intel.com>,
	Ashok Raj <ashok.raj@intel.com>,
	kvm@vger.kernel.org, rafael@kernel.org,
	David Airlie <airlied@linux.ie>,
	linux-pci@vger.kernel.org,
	Thierry Reding <thierry.reding@gmail.com>,
	Diana Craciun <diana.craciun@oss.nxp.com>,
	Dmitry Osipenko <digetx@gmail.com>, Will Deacon <will@kernel.org>,
	Stuart Yoder <stuyoder@gmail.com>,
	Jonathan Hunter <jonathanh@nvidia.com>,
	Chaitanya Kulkarni <kch@nvidia.com>,
	Dan Williams <dan.j.williams@intel.com>,
	Cornelia Huck <cohuck@redhat.com>,
	linux-kernel@vger.kernel.org, Li Yang <leoyang.li@nxp.com>,
	iommu@lists.linux-foundation.org,
	Jacob jun Pan <jacob.jun.pan@intel.com>,
	Daniel Vetter <daniel@ffwll.ch>,
	Robin Murphy <robin.murphy@arm.com>
Subject: Re: [PATCH v7 06/11] PCI: portdrv: Set driver_managed_dma
Date: Tue, 1 Mar 2022 10:54:09 +0800	[thread overview]
Message-ID: <80672557-59ab-8eb9-2fcf-d045ff52104b@linux.intel.com> (raw)
In-Reply-To: <20220228195628.GA515785@bhelgaas>

Hi Bjorn,

On 3/1/22 3:56 AM, Bjorn Helgaas wrote:
> On Mon, Feb 28, 2022 at 08:50:51AM +0800, Lu Baolu wrote:
>> If a switch lacks ACS P2P Request Redirect, a device below the switch can
>> bypass the IOMMU and DMA directly to other devices below the switch, so
>> all the downstream devices must be in the same IOMMU group as the switch
>> itself.
>>
>> The existing VFIO framework allows the portdrv driver to be bound to the
>> bridge while its downstream devices are assigned to user space. The
>> pci_dma_configure() marks the IOMMU group as containing only devices
>> with kernel drivers that manage DMA. Avoid this default behavior for the
>> portdrv driver in order for compatibility with the current VFIO usage.
> 
> It would be nice to explicitly say here how we can look at portdrv
> (and pci_stub) and conclude that ".driver_managed_dma = true" is safe.
> 
> Otherwise I won't know what kind of future change to portdrv might
> make it unsafe.

Fair enough. We can add below words:

We achieve this by setting ".driver_managed_dma = true" in pci_driver
structure. It is safe because the portdrv driver meets below criteria:

- This driver doesn't use DMA, as you can't find any related calls like
   pci_set_master() or any kernel DMA API (dma_map_*() and etc.).
- It doesn't use MMIO as you can't find ioremap() or similar calls. It's
   tolerant to userspace possibly also touching the same MMIO registers
   via P2P DMA access.

> 
>> Suggested-by: Jason Gunthorpe <jgg@nvidia.com>
>> Suggested-by: Kevin Tian <kevin.tian@intel.com>
>> Signed-off-by: Lu Baolu <baolu.lu@linux.intel.com>
>> Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
> 
> Acked-by: Bjorn Helgaas <bhelgaas@google.com>

Thank you!

Best regards,
baolu

> 
>> ---
>>   drivers/pci/pcie/portdrv_pci.c | 2 ++
>>   1 file changed, 2 insertions(+)
>>
>> diff --git a/drivers/pci/pcie/portdrv_pci.c b/drivers/pci/pcie/portdrv_pci.c
>> index 35eca6277a96..6b2adb678c21 100644
>> --- a/drivers/pci/pcie/portdrv_pci.c
>> +++ b/drivers/pci/pcie/portdrv_pci.c
>> @@ -202,6 +202,8 @@ static struct pci_driver pcie_portdriver = {
>>   
>>   	.err_handler	= &pcie_portdrv_err_handler,
>>   
>> +	.driver_managed_dma = true,
>> +
>>   	.driver.pm	= PCIE_PORTDRV_PM_OPS,
>>   };
>>   
>> -- 
>> 2.25.1
>>
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