[D][PATCH v2 0/3] Add kernel parameter 'pci=clearmsi' to clear MSI(X)s early on boot

[Mauricio Faria de Oliveira]

BugLink: https://bugs.launchpad.net/bugs/1797990

(Note: the patch set for Cosmic/Disco should be identical,
 as both trees' master-next branch is at the 4.18.0-11.12
 level, with some more patches in Cosmic, but doing Disco
 as well for process guidelines/completeness sake.)

[Changelog]
 * v2:
   - Reorder patch 1 as 3 to allow for the next change:
   - Gate the bus-scan differences with the cmdline option
     (patch 3 only).  Now all functional changes are gated.

[Impact]

 * A kexec/crash kernel might get stuck and fail to boot
   (for crash kernel, kdump fails to collect a crashdump)
   if a PCI device is buggy/stuck/looping and triggers a
   continuous flood of MSI(X) interrupts (that the kernel
   does not yet know about).

 * This fix allowed to obtain crashdumps when debugging a
   heavy-load scenario, in which a (heavy-loaded) network
   adapter wouldn't stop triggering MSI-X interrupts ever
   after panic()->kdump kicked in.

 * This fix disables MSI(X) in all PCI devices on early
   boot (this is OK as it's (re-)enabled normally later)
   with a kernel cmdline parameter (disabled by default).

[Test Case]

 * A synthetic test-case is not yet available, however,
   this particular system/workload triggered the problem
   consistently, and it was used for development/testing.

 * We'll update this bug once a synthetic test-case is
   available; we're working on patching QEMU for this.

 * $ dmesg | grep 'Clearing MSI'
   [    0.000000] Clearing MSI/MSI-X enable bits early in boot (quirk)

 * The comparison of 'dmesg -t | sort' has been reviewed
   between option disabled/enabled on boot & kexec modes,
   and only expected differences found (MHz, PIDs, MIPS).

[Regression Potential] 

 * The potential area for regressions is early boot,
   particularly effects of applying quirks during PCI
   bus scan, which is changed/broader w/ these patches.

 * However, all quirks are applied based on PCI ID
   matching, so would only apply if actually targeting
   a new device.

 * Moreover, the new quirk is only applied based on
   a kernel cmdline parameter that is disabled by
   default, which constraints even more when this
   is actually in effect.

[Other Info]
 
 * The patch series is still under review/discussion
   upstream, but it's relatively important for Ubuntu
   users at this point, and after internal discussions
   we decided to submit it for SRU.

 * These are links to the linux-pci archive with the
   patches [1, 2, 3]

   [1] [PATCH 1/3] x86/quirks: Scan all busses for early PCI quirks
       https://lore.kernel.org/linux-pci/20181018183721.27467-1-gpiccoli@canonical.com/

   [2] [PATCH 2/3] x86/PCI: Export find_cap() to be used in early PCI code
       https://lore.kernel.org/linux-pci/20181018183721.27467-2-gpiccoli@canonical.com/

   [3] [PATCH 3/3] x86/quirks: Add parameter to clear MSIs early on boot
       https://lore.kernel.org/linux-pci/20181018183721.27467-3-gpiccoli@canonical.com/

Guilherme G. Piccoli (3):
  UBUNTU: SAUCE: x86/PCI: Export find_cap() to be used in early PCI code
  UBUNTU: SAUCE: x86/quirks: Add parameter to clear MSIs early on boot
  UBUNTU: SAUCE: x86/quirks: Scan all busses for early PCI quirks

 .../admin-guide/kernel-parameters.txt         |  6 +++
 arch/x86/include/asm/pci-direct.h             |  2 +
 arch/x86/kernel/aperture_64.c                 | 30 +-------------
 arch/x86/kernel/early-quirks.c                | 41 +++++++++++++++++++
 arch/x86/pci/common.c                         |  4 ++
 arch/x86/pci/early.c                          | 25 +++++++++++
 6 files changed, 80 insertions(+), 28 deletions(-)

-- 
2.17.1