linux-kernel - Re: [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch

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Date:   Fri, 18 Dec 2020 11:20:45 -0800
From:   Dan Williams <dan.j.williams@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     "Weiny, Ira" <ira.weiny@...el.com>, Ingo Molnar <mingo@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>, X86 ML <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux Doc Mailing List <linux-doc@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        Linux MM <linux-mm@...ck.org>, linux-kselftest@...r.kernel.org,
        Greg KH <gregkh@...uxfoundation.org>
Subject: Re: [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch

On Fri, Dec 18, 2020 at 5:58 AM Thomas Gleixner <tglx@...utronix.de> wrote:
[..]
>   5) The DAX case which you made "work" with dev_access_enable() and
>      dev_access_disable(), i.e. with yet another lazy approach of
>      avoiding to change a handful of usage sites.
>
>      The use cases are strictly context local which means the global
>      magic is not used at all. Why does it exist in the first place?
>
>      Aside of that this global thing would never work at all because the
>      refcounting is per thread and not global.
>
>      So that DAX use case is just a matter of:
>
>         grant/revoke_access(DEV_PKS_KEY, READ/WRITE)
>
>      which is effective for the current execution context and really
>      wants to be a distinct READ/WRITE protection and not the magic
>      global thing which just has on/off. All usage sites know whether
>      they want to read or write.

I was tracking and nodding until this point. Yes, kill the global /
kmap() support, but if grant/revoke_access is not integrated behind
kmap_{local,atomic}() then it's not a "handful" of sites that need to
be instrumented it's 100s. Are you suggesting that "relaxed" mode
enforcement is a way to distribute the work of teaching driver writers
that they need to incorporate explicit grant/revoke-read/write in
addition to kmap? The entire reason PTE_DEVMAP exists was to allow
get_user_pages() for PMEM and not require every downstream-GUP code
path to specifically consider whether it was talking to PMEM or RAM
pages, and certainly not whether they were reading or writing to it.