linux-kernel - [PATCH 7/9] x86/pci: Comment pci_mmconfig

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Message-Id: <20231121183643.249006-8-helgaas@kernel.org>
Date:   Tue, 21 Nov 2023 12:36:41 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     linux-pci@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Dan Williams <dan.j.williams@...el.com>,
        Kan Liang <kan.liang@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Giovanni Cabiddu <giovanni.cabiddu@...el.com>,
        Yunying Sun <yunying.sun@...el.com>,
        Tomasz Pala <gotar@...anet.pl>,
        Sebastian Manciulea <manciuleas@...tonmail.com>,
        linux-kernel@...r.kernel.org, Bjorn Helgaas <bhelgaas@...gle.com>
Subject: [PATCH 7/9] x86/pci: Comment pci_mmconfig_insert() obscure MCFG dependency

From: Bjorn Helgaas <bhelgaas@...gle.com>

In pci_mmconfig_insert(), there's no reference to "addr" between locking
pci_mmcfg_lock and testing "addr", so it *looks* like we should move the
test before the lock.

But 07f9b61c3915 ("x86/PCI: MMCONFIG: Check earlier for MMCONFIG region at
address zero") did that, which broke things by returning -EINVAL when
"addr" is zero instead of -EEXIST.

So 07f9b61c3915 was reverted by 67d470e0e171 ("Revert "x86/PCI: MMCONFIG:
Check earlier for MMCONFIG region at address zero"").

Add a comment about this issue to prevent it from happening again.

Signed-off-by: Bjorn Helgaas <bhelgaas@...gle.com>
---
 arch/x86/pci/mmconfig-shared.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/pci/mmconfig-shared.c b/arch/x86/pci/mmconfig-shared.c
index b36c10e86505..459e95782bb1 100644
--- a/arch/x86/pci/mmconfig-shared.c
+++ b/arch/x86/pci/mmconfig-shared.c
@@ -786,6 +786,10 @@ int pci_mmconfig_insert(struct device *dev, u16 seg, u8 start, u8 end,
 		return -EEXIST;
 	}
 
+	/*
+	 * Don't move earlier; we must return -EEXIST, not -EINVAL, if
+	 * pci_mmconfig_lookup() finds something
+	 */
 	if (!addr) {
 		mutex_unlock(&pci_mmcfg_lock);
 		return -EINVAL;
-- 
2.34.1