Re: [PATCH V7 9/9] PCI/P2PDMA: Add a 10-Bit Tag check in P2PDMA

[Dongdong Liu <liudongdong3@huawei.com>]

On 2021/8/6 2:12, Bjorn Helgaas wrote:
> On Wed, Aug 04, 2021 at 09:47:08PM +0800, Dongdong Liu wrote:
>> Add a 10-Bit Tag check in the P2PDMA code to ensure that a device with
>> 10-Bit Tag Requester doesn't interact with a device that does not
>> support 10-BIT Tag Completer. Before that happens, the kernel should
>> emit a warning. "echo 0 > /sys/bus/pci/devices/.../10bit_tag" to
>> disable 10-BIT Tag Requester for PF device.
>> "echo 0 > /sys/bus/pci/devices/.../sriov_vf_10bit_tag_ctl" to disable
>> 10-BIT Tag Requester for VF device.
>
> s/10-BIT/10-Bit/ several times.
Will fix.
>
> Add blank lines between paragraphs.
Will fix.
>
>> Signed-off-by: Dongdong Liu <liudongdong3@huawei.com>
>> ---
>>  drivers/pci/p2pdma.c | 40 ++++++++++++++++++++++++++++++++++++++++
>>  1 file changed, 40 insertions(+)
>>
>> diff --git a/drivers/pci/p2pdma.c b/drivers/pci/p2pdma.c
>> index 50cdde3..948f2be 100644
>> --- a/drivers/pci/p2pdma.c
>> +++ b/drivers/pci/p2pdma.c
>> @@ -19,6 +19,7 @@
>>  #include <linux/random.h>
>>  #include <linux/seq_buf.h>
>>  #include <linux/xarray.h>
>> +#include "pci.h"
>>
>>  enum pci_p2pdma_map_type {
>>  	PCI_P2PDMA_MAP_UNKNOWN = 0,
>> @@ -410,6 +411,41 @@ static unsigned long map_types_idx(struct pci_dev *client)
>>  		(client->bus->number << 8) | client->devfn;
>>  }
>>
>> +static bool check_10bit_tags_vaild(struct pci_dev *a, struct pci_dev *b,
>
> s/vaild/valid/
>
> Or maybe s/valid/safe/ or s/valid/supported/, since "valid" isn't
> quite the right word here.  We want to know whether the source is
> enabled to generate 10-bit tags, and if so, whether the destination
> can handle them.
>
> "if (check_10bit_tags_valid())" does not make sense because
> "check_10bit_tags_valid()" is not a question with a yes/no answer.
>
> "10bit_tags_valid()" *might* be, because "if (10bit_tags_valid())"
> makes sense.  But I don't think you can start with a digit.
>
> Or maybe you want to invert the sense, e.g.,
> "10bit_tags_unsupported()", since that avoids negation at the caller:
>
>   if (10bit_tags_unsupported(a, b) ||
>       10bit_tags_unsupported(b, a))
>         map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
Good suggestion. add a pci_ prefix.

if (pci_10bit_tags_unsupported(a, b) ||
     pci_10bit_tags_unsupported(b, a))
	map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;

> Doesn't this patch need to be at the very beginning, before you start
> enabling 10-bit tags?  Otherwise there's a hole in the middle where we
> enable them and P2P DMA might break.
Yes, will do.
>
>> +				   bool verbose)
>> +{
>> +	bool req;
>> +	bool comp;
>> +	u16 ctl2;
>> +
>> +	if (a->is_virtfn) {
>> +#ifdef CONFIG_PCI_IOV
>> +		req = !!(a->physfn->sriov->ctrl &
>> +			 PCI_SRIOV_CTRL_VF_10BIT_TAG_REQ_EN);
>> +#endif
>> +	} else {
>> +		pcie_capability_read_word(a, PCI_EXP_DEVCTL2, &ctl2);
>> +		req = !!(ctl2 & PCI_EXP_DEVCTL2_10BIT_TAG_REQ_EN);
>> +	}
>> +
>> +	comp = !!(b->pcie_devcap2 & PCI_EXP_DEVCAP2_10BIT_TAG_COMP);
>> +	if (req && (!comp)) {
>> +		if (verbose) {
>> +			pci_warn(a, "cannot be used for peer-to-peer DMA as 10-Bit Tag Requester enable is set in device (%s), but peer device (%s) does not support the 10-Bit Tag Completer\n",
>> +				 pci_name(a), pci_name(b));
>
> No point in printing pci_name(a) twice.  pci_warn() prints it already;
> that should be enough.
Will fix.
>
> I think you can simplify this a little, e.g.,
>
>   if (!req)           /* 10-bit tags not enabled on requester */
>     return true;
>
>   if (comp)           /* completer can handle anything */
>     return true;
>
>   /* error case */
>   if (!verbose)
>     return false;
>
>   pci_warn(...);
>   return false;

Good point, this will make code more clean and readable.

Thanks,
Dongdong
>
>> +			if (a->is_virtfn)
>> +				pci_warn(a, "to disable 10-Bit Tag Requester for this device, echo 0 > /sys/bus/pci/devices/%s/sriov_vf_10bit_tag_ctl\n",
>> +					 pci_name(a));
>> +			else
>> +				pci_warn(a, "to disable 10-Bit Tag Requester for this device, echo 0 > /sys/bus/pci/devices/%s/10bit_tag\n",
>> +					 pci_name(a));
>> +		}
>> +		return false;
>> +	}
>> +
>> +	return true;
>> +}
>> +
>>  /*
>>   * Calculate the P2PDMA mapping type and distance between two PCI devices.
>>   *
>> @@ -532,6 +568,10 @@ calc_map_type_and_dist(struct pci_dev *provider, struct pci_dev *client,
>>  		map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
>>  	}
>>  done:
>> +	if (!check_10bit_tags_vaild(client, provider, verbose) ||
>> +	    !check_10bit_tags_vaild(provider, client, verbose))
>> +		map_type = PCI_P2PDMA_MAP_NOT_SUPPORTED;
>> +
>>  	rcu_read_lock();
>>  	p2pdma = rcu_dereference(provider->p2pdma);
>>  	if (p2pdma)
>> --
>> 2.7.4
>>
> .
>