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 22:06:24 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dan Williams <dan.j.williams@...el.com>
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 11:20, Dan Williams wrote:
> 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.

kmap_local() is fine. That can work automatically because it's strict
local to the context which does the mapping.

kmap() is dubious because it's a 'global' mapping as dictated per
HIGHMEM. So doing the RELAXED mode for kmap() is sensible I think to
identify cases where the mapped address is really handed to a different
execution context. We want to see those cases and analyse whether this
can't be solved in a different way. That's why I suggested to do a
warning in that case.

Also vs. the DAX use case I really meant the code in fs/dax and
drivers/dax/ itself which is handling this via dax_read_[un]lock.

Does that make more sense?

Thanks,

        tglx