linux-kernel - Re: DAX filesystem support on ARMv8

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Date:   Sun, 1 Dec 2019 13:54:45 -0500
From:   Jon Masters <jcm@...masters.org>
To:     Bharat Kumar Gogada <bharatku@...inx.com>,
        Matthew Wilcox <willy@...radead.org>,
        Dan Williams <dan.j.williams@...el.com>
Cc:     "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 11/14/19 1:54 AM, Bharat Kumar Gogada wrote:
>>
>> 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.
> 
> Thanks a lot Wilcox and Dan for clarification.
> So the above restriction only applies to ARM architectures with VIVT caches and not
> for VIPT caches.

VMSAv8-64 (Armv8) requires that data caches behave as if they were PIPT. 
Meaning there is not a situation as described above.

Jon.

-- 
Computer Architect