linux-kernel - Re: [PATCH v9 07/38] x86/mm: Remove phys_to

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Date:   Tue, 11 Jul 2017 10:35:17 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Brian Gerst <brgerst@...il.com>
Cc:     Tom Lendacky <thomas.lendacky@....com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-efi@...r.kernel.org, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>,
        kexec@...ts.infradead.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        kasan-dev <kasan-dev@...glegroups.com>, xen-devel@...ts.xen.org,
        Linux-MM <linux-mm@...ck.org>,
        "open list:IOMMU DRIVERS" <iommu@...ts.linux-foundation.org>,
        Brijesh Singh <brijesh.singh@....com>,
        Toshimitsu Kani <toshi.kani@....com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Alexander Potapenko <glider@...gle.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Larry Woodman <lwoodman@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Joerg Roedel <joro@...tes.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        Dave Young <dyoung@...hat.com>, Rik van Riel <riel@...hat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Juergen Gross <jgross@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v9 07/38] x86/mm: Remove phys_to_virt() usage in ioremap()

On Tue, Jul 11, 2017 at 6:58 AM, Brian Gerst <brgerst@...il.com> wrote:
> On Mon, Jul 10, 2017 at 3:50 PM, Tom Lendacky <thomas.lendacky@....com> wrote:
>> On 7/8/2017 7:57 AM, Brian Gerst wrote:
>>> On Fri, Jul 7, 2017 at 9:39 AM, Tom Lendacky <thomas.lendacky@....com>
>>
>> I originally had a check for SME here in a previous version of the
>> patch.  Thomas Gleixner recommended removing the check so that the code
>> path was always exercised regardless of the state of SME in order to
>> better detect issues:
>>
>> http://marc.info/?l=linux-kernel&m=149803067811436&w=2
>
> Looking a bit closer, this shortcut doesn't set the caching
> attributes.  So it's probably best to get rid of it anyways.  Also
> note, there is a corresponding check in iounmap().

Could that cause regressions if a driver relies on (write-through)
cacheable access to the VGA frame buffer RAM or an read-only
cached access to an option ROM but now gets uncached access?

I also tried to find out whether we can stop mapping the ISA MMIO
area into the linear mapping, but at least the VGA code uses
VGA_MAP_MEM() to get access to the same pointers. I'm pretty
sure this got copied incorrectly into most other architectures, but
it is definitely still used on x86 with vga16fb/vgacon/mdacon.

On the plus side, I see that removing this code path will end up
restoring MMIOTRACE support for the ISA MMIO range that was
apparently removed by accident in commit d61fc44853f4
("x86: mmiotrace, preview 2") in linux-2.6.27.

      Arnd