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From:   Matthew Garrett <mjg59@google.com>
Date:   Tue, 3 Dec 2019 11:40:23 -0800
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Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Laszlo Ersek <lersek@redhat.com>,
        linux-efi <linux-efi@vger.kernel.org>,
        "the arch/x86 maintainers" <x86@kernel.org>,
        linux-pci <linux-pci@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Tue, Dec 3, 2019 at 3:54 AM Ard Biesheuvel <ard.biesheuvel@linaro.org> wrote:

> There is no reason this shouldn't apply to ARM, but disabling bus
> mastering like that before the drivers themselves get a chance to do
> so is likely to cause trouble. Network devices or storage controllers
> that are still running and have live descriptor rings in DMA memory
> shouldn't get the rug pulled from under their feet like that by
> blindly disabling the BM attribute on all root ports before their
> drivers have had the opportunity to do this cleanly.

Yes, whether this causes problems is going to be influenced by the
behaviour of the hardware on the system. That's why I'm not defaulting
it to being enabled :)

> One trick we implemented in EDK2 for memory encryption was to do the
> following (Laszlo, mind correcting me here if I am remembering this
> wrong?)
> - create an event X
> - register an AtExitBootServices event that signals event X in its handler
> - in the handler of event X, iterate over all PPBs to clear the bus
> master attribute
> - for bonus points, do the same for the PCIe devices themselves,
> because root ports are known to exist that entirely ignore the BM
> attribute
>
> This way, event X should get handled after all the drivers' EBS event
> handlers have been called.

Can we guarantee that this happens before IOMMU state teardown? I
don't think there's a benefit to clearing the bit on endpoint devices,
if they're malicious they're just going to turn it back on anyway.