[PATCH v3 5/9] perf/cxl: Accept an overflow interrupt on MSI message number 0

cxl_pmu_probe() rejects the PMU when info->msi_vec <= 0, but that field
holds the MSI/MSI-X message number the device signals overflow on. The
field is 0-based, and -1 means no interrupt support. Message number 0 is
valid and pci_irq_vector() takes a 0-based index, so a compliant device
signalling on the first vector fails to probe.

Reject only the no-interrupt case, matching how the CXL mailbox and event
interrupts handle it.

Fixes: 5d7107c72796 ("perf: CXL Performance Monitoring Unit driver")
Reported-by: [email protected]
Closes: https://sashiko.dev/#/patchset/[email protected]?part=1
Assisted-by: Claude:claude-opus-4-8
Reviewed-by: Jonathan Cameron <[email protected]>
Reviewed-by: Davidlohr Bueso <[email protected]>
Signed-off-by: Dave Jiang <[email protected]>
---
v3:
- Rebase on the preceding patch, so this tests info->msi_vec rather than
  info->irq. The fix is unchanged and the hunk is still one line.
- Not guarding against pci_irq_vector()'s INTx fallback for vector 0, which
  it returns when neither MSI nor MSI-X is enabled. A v3 draft added a
  pci_dev_msi_enabled() check for that, but a modern CXL device wiring up
  INTx is not a realistic prospect, and the consequence otherwise is a PMU
  that counts nothing rather than anything unsafe. Left alone to keep this
  a one-liner.
---
 drivers/perf/cxl_pmu.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/perf/cxl_pmu.c b/drivers/perf/cxl_pmu.c
index 56e6ae7bebf7..c9e30cb149df 100644
--- a/drivers/perf/cxl_pmu.c
+++ b/drivers/perf/cxl_pmu.c
@@ -875,7 +875,7 @@ static int cxl_pmu_probe(struct device *dev)
 		.capabilities = PERF_PMU_CAP_NO_EXCLUDE,
 	};
 
-	if (info->msi_vec <= 0)
+	if (info->msi_vec < 0)
 		return -EINVAL;
 
 	rc = pci_irq_vector(pdev, info->msi_vec);
-- 
2.55.0