linux-kernel - Re: [PATCH v3 7/7] mm/memremap: Set caching mode for PCI P2PDMA memory to WC

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Message-ID: <6e2591ad-4605-aba4-c7b8-2345940e91b4@deltatee.com>
Date:   Mon, 2 Mar 2020 14:20:07 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        linux-ia64@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Linux-sh <linux-sh@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, Linux MM <linux-mm@...ck.org>,
        Michal Hocko <mhocko@...nel.org>,
        David Hildenbrand <david@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Christoph Hellwig <hch@....de>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Eric Badger <ebadger@...aio.com>,
        Jason Gunthorpe <jgg@...pe.ca>
Subject: Re: [PATCH v3 7/7] mm/memremap: Set caching mode for PCI P2PDMA
 memory to WC



On 2020-02-29 3:47 p.m., Dan Williams wrote:
> On Fri, Feb 21, 2020 at 10:25 AM Logan Gunthorpe <logang@...tatee.com> wrote:
>>
>> PCI BAR IO memory should never be mapped as WB, however prior to this
>> the PAT bits were set WB and it was typically overridden by MTRR
>> registers set by the firmware.
>>
>> Set PCI P2PDMA memory to be WC (writecombining) as the only current
>> user (the NVMe CMB) was originally mapped WC before the P2PDMA code
>> replaced the mapping with devm_memremap_pages().
> 
> Will the change to UC regress this existing use case?

I don't think so. They've been essentially mapped UC for a long time now
(since the P2PDMA patch set was merged) and nobody has complained.