linux-kernel - Re: [PATCH v2] PCI: hv: fix reading of PCI_INTERRUPT

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Date: Mon, 1 Jul 2024 12:20:53 -0500
From: Bjorn Helgaas <helgaas@...nel.org>
To: Wei Liu <wei.liu@...nel.org>
Cc: Linux on Hyper-V List <linux-hyperv@...r.kernel.org>, stable@...nel.org,
	"K. Y. Srinivasan" <kys@...rosoft.com>,
	Haiyang Zhang <haiyangz@...rosoft.com>,
	Dexuan Cui <decui@...rosoft.com>,
	Lorenzo Pieralisi <lpieralisi@...nel.org>,
	Krzysztof Wilczyński <kw@...ux.com>,
	Rob Herring <robh@...nel.org>, Bjorn Helgaas <bhelgaas@...gle.com>,
	Jake Oshins <jakeo@...rosoft.com>,
	"open list:PCI NATIVE HOST BRIDGE AND ENDPOINT DRIVERS" <linux-pci@...r.kernel.org>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] PCI: hv: fix reading of PCI_INTERRUPT_PIN

On Mon, Jul 01, 2024 at 06:16:18AM +0000, Wei Liu wrote:
> On Wed, Jun 26, 2024 at 10:10:39AM -0500, Bjorn Helgaas wrote:
> > 1) Capitalize subject to match history
> 
> What do you mean here? I got the "PCI: hv: ..." format from recent
> commits. "PCI" is capitalized. You want to to capitalize "fix"?

Yes.  Look at the history:

  $ git log --oneline --no-merges drivers/pci/controller/pci-hyperv.c
  b5ff74c1ef50 PCI: hv: Fix ring buffer size calculation
  07e8f88568f5 x86/apic: Drop apic::delivery_mode
  f741bcadfe52 PCI: hv: Annotate struct hv_dr_state with __counted_by
  04bbe863241a PCI: hv: Fix a crash in hv_pci_restore_msi_msg() during hibernation
  067d6ec7ed5b PCI: hv: Add a per-bus mutex state_lock
  a847234e24d0 Revert "PCI: hv: Fix a timing issue which causes kdump to fail occasionally"
  add9195e69c9 PCI: hv: Remove the useless hv_pcichild_state from struct hv_pci_dev
  2738d5ab7929 PCI: hv: Fix a race condition in hv_irq_unmask() that can cause panic
  ...

> > 2) Say something more specific than "fix reading ..."
> > 
> > Apparently this returns garbage in some case where you want to return
> > zero?
> 
> Yes. *val is not changed in the old code, so garbage is returned.
> 
> Here is the updated commit message. I can resend once you confirm you're
> happy with it.
> 
>     PCI: hv: Fix reading of PCI_INTERRUPT_PIN

Maybe:

  PCI: hv: Return zero, not garbage, when reading PCI_INTERRUPT_PIN

>     The intent of the code snippet is to always return 0 for both
>     PCI_INTERRUPT_LINE and PCI_INTERRUPT_PIN.
> 
>     The check misses PCI_INTERRUPT_PIN. This patch fixes that.
> 
>     This is discovered by this call in VFIO:
> 
>         pci_read_config_byte(vdev->pdev, PCI_INTERRUPT_PIN, &pin);
> 
>     The old code does not set *val to 0 because it misses the check for
>     PCI_INTERRUPT_PIN. Garbage is returned in this case.
> 
>     Fixes: 4daace0d8ce8 ("PCI: hv: Add paravirtual PCI front-end for Microsoft Hyper-V VMs")
>     Cc: stable@...nel.org
>     Signed-off-by: Wei Liu <wei.liu@...nel.org>

Looks fine.