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

On 2021/8/6 2:12, Bjorn Helgaas wrote:

On Wed, Aug 04, 2021 at 09:47:08PM +0800, Dongdong Liu wrote:

quoted

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.

quoted

Signed-off-by: Dongdong Liu <redacted>
---
 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.

quoted

+				   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

quoted

+			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

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