linux-kernel - Re: [PATCH v3 0/7] Allow setting caching mode in arch_add

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Message-ID: <e8781f85-3fc7-b9ce-c751-606803cbdc77@deltatee.com>
Date:   Thu, 27 Feb 2020 10:21:50 -0700
From:   Logan Gunthorpe <logang@...tatee.com>
To:     Jason Gunthorpe <jgg@...pe.ca>
Cc:     linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-ia64@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-s390@...r.kernel.org, linux-sh@...r.kernel.org,
        platform-driver-x86@...r.kernel.org, linux-mm@...ck.org,
        Dan Williams <dan.j.williams@...el.com>,
        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>
Subject: Re: [PATCH v3 0/7] Allow setting caching mode in arch_add_memory()
 for P2PDMA



On 2020-02-27 10:17 a.m., Jason Gunthorpe wrote:
>> Instead of this, this series proposes a change to arch_add_memory()
>> to take the pgprot required by the mapping which allows us to
>> explicitly set pagetable entries for P2PDMA memory to WC.
> 
> Is there a particular reason why WC was selected here? I thought for
> the p2pdma cases there was no kernel user that touched the memory?

Yes, that's correct. I choose WC here because the existing users are
registering memory blocks without side effects which fit the WC
semantics well.

> I definitely forsee devices where we want UC instead.

Yes. My expectation is that once we have a kernel user that needs this,
we'd wire the option through struct dev_pagemap so the caller can choose
the mapping that makes sense.

Logan