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From:   Andy Lutomirski <luto@amacapital.net>
Date:   Wed, 4 Dec 2019 11:50:31 -0800
Message-ID: <CALCETrWZwN-R=He2s1DLet8iOxB_AbuSGOJ3y7zW=qUmx33C=A@mail.gmail.com>
Subject: Re: [PATCH] [EFI,PCI] Allow disabling PCI busmastering on bridges
 during boot
To:     Matthew Garrett <mjg59@google.com>
Cc:     linux-efi <linux-efi@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        X86 ML <x86@kernel.org>, linux-pci <linux-pci@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
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On Tue, Dec 3, 2019 at 11:41 AM Matthew Garrett <mjg59@google.com> wrote:
>
> On Tue, Dec 3, 2019 at 7:30 AM Andy Lutomirski <luto@amacapital.net> wrote:
>
> > Would a similar patch apply to non-EFI boot?  That is, in a BIOS boot,
> > is busmastering on when the kernel is loaded?
>
> It's only relevant where firmware configures the IOMMU but then
> removes that configuration before handing control to the OS. I'm not
> aware of that happening anywhere other than EFI.

Wouldn't it also be applicable in the much simpler case where the
firmware hands over control with no IOMMU configured but also with the
busmastering bit cleared.  Does firmware do this?  Does the kernel
currently configure the iOMMU before enabling busmastering?

--Andy