linux-kernel - Re: [PATCH] PCI/MSI: Avoid torn updates to MSI pairs

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Date:   Fri, 17 Jan 2020 16:27:37 -0800
From:   Evan Green <evgreen@...omium.org>
To:     Bjorn Helgaas <bhelgaas@...gle.com>
Cc:     linux-pci@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PCI/MSI: Avoid torn updates to MSI pairs

On Thu, Jan 16, 2020 at 1:31 PM Evan Green <evgreen@...omium.org> wrote:
>
> __pci_write_msi_msg() updates three registers in the device: address
> high, address low, and data. On x86 systems, address low contains
> CPU targeting info, and data contains the vector. The order of writes
> is address, then data.
>
> This is problematic if an interrupt comes in after address has
> been written, but before data is updated, and the SMP affinity of
> the interrupt is changing. In this case, the interrupt targets the
> wrong vector on the new CPU.
>
> This case is pretty easy to stumble into using xhci and CPU hotplugging.
> Create a script that targets interrupts at a set of cores and then
> offlines those cores. Put some stress on USB, and then watch xhci lose
> an interrupt and die.
>
> Avoid this by disabling MSIs during the update.
>
> Signed-off-by: Evan Green <evgreen@...omium.org>

Note to reviewers: I posted a v2 of this patch with some improvements here:
https://lore.kernel.org/lkml/20200117162444.v2.1.I9c7e72144ef639cc135ea33ef332852a6b33730f@changeid/T/#u