linux-kernel - Re: DAX filesystem support on ARMv8

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Date:   Tue, 12 Nov 2019 14:02:12 -0800
From:   Matthew Wilcox <willy@...radead.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Bharat Kumar Gogada <bharatku@...inx.com>,
        "linux-nvdimm@...ts.01.org" <linux-nvdimm@...ts.01.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "viro@...iv.linux.org.uk" <viro@...iv.linux.org.uk>,
        "jack@...e.cz" <jack@...e.cz>
Subject: Re: DAX filesystem support on ARMv8

On Tue, Nov 12, 2019 at 09:15:18AM -0800, Dan Williams wrote:
> On Mon, Nov 11, 2019 at 6:12 PM Bharat Kumar Gogada <bharatku@...inx.com> wrote:
> >
> > Hi All,
> >
> > As per Documentation/filesystems/dax.txt
> >
> > The DAX code does not work correctly on architectures which have virtually
> > mapped caches such as ARM, MIPS and SPARC.
> >
> > Can anyone please shed light on dax filesystem issue w.r.t ARM architecture ?
> 
> The concern is VIVT caches since the kernel will want to flush pmem
> addresses with different virtual addresses than what userspace is
> using. As far as I know, ARMv8 has VIPT caches, so should not have an
> issue. Willy initially wrote those restrictions, but I am assuming
> that the concern was managing the caches in the presence of virtual
> aliases.

The kernel will also access data at different virtual addresses from
userspace.  So VIVT CPUs will be mmap/read/write incoherent, as well as
being flush incoherent.