Re: [PATCH V2] PCI: Enable PASID when End-to-End TLP is supported by all bridges

[Sinan Kaya]

On 6/29/2018 8:49 PM, Bjorn Helgaas wrote:
> On Tue, Jun 19, 2018 at 10:14:46PM -0400, Sinan Kaya wrote:
>> A PCIe endpoint carries the process address space identifier (PASID) in
>> the TLP prefix as part of the memory read/write transaction. The address
>> information in the TLP is relevant only for a given PASID context.
>>
>> An IOMMU takes PASID value and the address information from the
>> TLP to look up the physical address in the system.
>>
>> If a bridge drops the TLP prefix, the translation agent can resolve the
>> address to an incorrect location and cause data corruption. Prevent
>> this condition by requiring End-to-End TLP prefix to be supported on the
>> entire data path between the endpoint and the root port.
> 
> PASID is an End-End TLP Prefix (PCIe r4.0, sec 6.20).  Sec 2.2.10.2 says
> 
>   It is an error to receive a TLP with an End-End TLP Prefix by a
>   Receiver that does not support End-End TLP Prefixes. A TLP in
>   violation of this rule is handled as a Malformed TLP. This is a
>   reported error associated with the Receiving Port (see Section 6.2).
> 
> So I agree that we shouldn't enable PASID in an endpoint unless all
> the switch ports leading to it support End-End prefixes.  But I don't
> see how a bridge can drop a prefix and cause data corruption -- if it
> doesn't support End-End prefixes, shouldn't the bridge raise a
> Malformed TLP error instead of forwarding the TLP?

It should under normal circumstances. 

I remember reading that most PCIe switches don't support TLP prefixes.
I don't know if it is because of buggy behavior or if it is just plain
unsupported while dropping the request as Malformed TLP.

I was trying to be proactive and not enable PASID if the entire path
is incapable.

> 
>> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
>> ---
>>  drivers/pci/ats.c             |  9 +++++++++
>>  drivers/pci/probe.c           | 17 +++++++++++++++++
>>  include/linux/pci.h           |  1 +
>>  include/uapi/linux/pci_regs.h |  1 +
>>  4 files changed, 28 insertions(+)
>>
>> diff --git a/drivers/pci/ats.c b/drivers/pci/ats.c
>> index 4923a2a..e1b2e6d 100644
>> --- a/drivers/pci/ats.c
>> +++ b/drivers/pci/ats.c
>> @@ -268,6 +268,7 @@ EXPORT_SYMBOL_GPL(pci_reset_pri);
>>  int pci_enable_pasid(struct pci_dev *pdev, int features)
>>  {
>>  	u16 control, supported;
>> +	struct pci_dev *bridge;
>>  	int pos;
>>  
>>  	if (WARN_ON(pdev->pasid_enabled))
>> @@ -277,6 +278,14 @@ int pci_enable_pasid(struct pci_dev *pdev, int features)
>>  	if (!pos)
>>  		return -EINVAL;
>>  
>> +	bridge = pci_upstream_bridge(pdev);
>> +	while (bridge) {
>> +		if (!bridge->eetlp_prefix)
>> +			return -EINVAL;
>> +
>> +		bridge = pci_upstream_bridge(bridge);
>> +	}
> 
> I was hoping to avoid even this loop by having the eetlp_prefix bit
> indicate that "End-End TLP Prefixes are supported from the Root Port
> to here".
> 

I see.

>>  	pci_read_config_word(pdev, pos + PCI_PASID_CAP, &supported);
>>  	supported &= PCI_PASID_CAP_EXEC | PCI_PASID_CAP_PRIV;
>>  
>> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
>> index ac876e3..a7f7ac1 100644
>> --- a/drivers/pci/probe.c
>> +++ b/drivers/pci/probe.c
>> @@ -2042,6 +2042,22 @@ static void pci_configure_ltr(struct pci_dev *dev)
>>  #endif
>>  }
>>  
>> +static void pci_configure_eetlp_prefix(struct pci_dev *dev)
>> +{
>> +#ifdef CONFIG_PCI_PASID
>> +	u32 cap;
>> +
>> +	if (!pci_is_pcie(dev))
>> +		return;
>> +
>> +	pcie_capability_read_dword(dev, PCI_EXP_DEVCAP2, &cap);
>> +	if (!(cap & PCI_EXP_DEVCAP2_E2ETLP))
>> +		return;
>> +
>> +	dev->eetlp_prefix = 1;
> 
> I.e., here we would do:
> 
>   if (pci_pcie_type(dev) == PCI_EXP_TYPE_ROOT_PORT)
>     dev->eetlp_prefix_path = 1;
>   else {
>     bridge = pci_upstream_bridge(dev);
>     if (bridge && bridge->eetlp_prefix_path)
>       dev->eetlp_prefix_path = 1;
>   }

Sure, let me make the changes and post a new version.

> 
>> +#endif
>> +}
>> +
>>  static void pci_configure_device(struct pci_dev *dev)
>>  {
>>  	struct hotplug_params hpp;
>> @@ -2051,6 +2067,7 @@ static void pci_configure_device(struct pci_dev *dev)
>>  	pci_configure_extended_tags(dev, NULL);
>>  	pci_configure_relaxed_ordering(dev);
>>  	pci_configure_ltr(dev);
>> +	pci_configure_eetlp_prefix(dev);
>>  
>>  	memset(&hpp, 0, sizeof(hpp));
>>  	ret = pci_get_hp_params(dev, &hpp);
>> diff --git a/include/linux/pci.h b/include/linux/pci.h
>> index 340029b..cf88d47 100644
>> --- a/include/linux/pci.h
>> +++ b/include/linux/pci.h
>> @@ -350,6 +350,7 @@ struct pci_dev {
>>  	unsigned int	ltr_path:1;	/* Latency Tolerance Reporting
>>  					   supported from root to here */
>>  #endif
>> +	unsigned int	eetlp_prefix:1;	/* End-to-End TLP Prefix */
>>  
>>  	pci_channel_state_t error_state;	/* Current connectivity state */
>>  	struct device	dev;			/* Generic device interface */
>> diff --git a/include/uapi/linux/pci_regs.h b/include/uapi/linux/pci_regs.h
>> index 4da87e2..a617ab2 100644
>> --- a/include/uapi/linux/pci_regs.h
>> +++ b/include/uapi/linux/pci_regs.h
>> @@ -636,6 +636,7 @@
>>  #define  PCI_EXP_DEVCAP2_OBFF_MASK	0x000c0000 /* OBFF support mechanism */
>>  #define  PCI_EXP_DEVCAP2_OBFF_MSG	0x00040000 /* New message signaling */
>>  #define  PCI_EXP_DEVCAP2_OBFF_WAKE	0x00080000 /* Re-use WAKE# for OBFF */
>> +#define PCI_EXP_DEVCAP2_E2ETLP		0x00200000 /* End-to-End TLP Prefix */
> 
> It looks like lspci doesn't decode this bit (and several others in
> DevCap2).  Would you be interested in adding that?  The source is at
> git://git.kernel.org/pub/scm/utils/pciutils/pciutils.git
> 
>>  #define PCI_EXP_DEVCTL2		40	/* Device Control 2 */
>>  #define  PCI_EXP_DEVCTL2_COMP_TIMEOUT	0x000f	/* Completion Timeout Value */
>>  #define  PCI_EXP_DEVCTL2_COMP_TMOUT_DIS	0x0010	/* Completion Timeout Disable */
>> -- 
>> 2.7.4
>>
>>
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>> linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
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> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.