linux-kernel - Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR region in VMA

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Date:   Wed, 5 May 2021 19:02:31 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Vikram Sethi <vsethi@...dia.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Mark Kettenis <mark.kettenis@...all.nl>,
        Marc Zyngier <maz@...nel.org>,
        Shanker Donthineni <sdonthineni@...dia.com>,
        "christoffer.dall@....com" <christoffer.dall@....com>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "kvmarm@...ts.cs.columbia.edu" <kvmarm@...ts.cs.columbia.edu>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jason Sequeira <jsequeira@...dia.com>
Subject: Re: [RFC 1/2] vfio/pci: keep the prefetchable attribute of a BAR
 region in VMA

On Tue, May 04, 2021 at 09:30:05AM +0100, Will Deacon wrote:
> On Mon, May 03, 2021 at 10:03:59PM +0000, Vikram Sethi wrote:
> > Will/Catalin, perhaps you could explain your thought process on why you chose
> > Normal NC for ioremap_wc on the armv8 linux port instead of Device GRE or other
> > Device Gxx. 
> 
> I think a combination of: compatibility with 32-bit Arm, the need to
> support unaligned accesses and the potential for higher performance.

IIRC the _wc suffix also matches the pgprot_writecombine() used by some
drivers to map a video framebuffer into user space. Accesses to the
framebuffer are not guaranteed to be aligned (memset/memcpy don't ensure
alignment on arm64 and the user doesn't have a memset_io or memcpy_toio).

> Furthermore, ioremap() already gives you a Device memory type, and we're
> tight on MAIR space.

We have MT_DEVICE_GRE currently reserved though no in-kernel user, we
might as well remove it.

-- 
Catalin