linux-kernel - Re: [PATCH v3 00/10] EFI Specific Purpose Memory Support

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Date:   Fri, 7 Jun 2019 13:37:08 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Dave Jiang <dave.jiang@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Keith Busch <keith.busch@...el.com>,
        kbuild test robot <lkp@...el.com>,
        Andy Shevchenko <andy@...radead.org>,
        Borislav Petkov <bp@...en8.de>,
        Vishal Verma <vishal.l.verma@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Matthew Wilcox <willy@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, Len Brown <lenb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        Darren Hart <dvhart@...radead.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>,
        linux-efi <linux-efi@...r.kernel.org>
Subject: Re: [PATCH v3 00/10] EFI Specific Purpose Memory Support

On Fri, Jun 7, 2019 at 12:57 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 6/7/19 12:27 PM, Dan Williams wrote:
> > In support of optionally allowing either application-exclusive and
> > core-kernel-mm managed access to differentiated memory, claim
> > EFI_MEMORY_SP ranges for exposure as device-dax instances by default.
> > Such instances can be directly owned / mapped by a
> > platform-topology-aware application. Alternatively, with the new kmem
> > facility [4], the administrator has the option to instead designate that
> > those memory ranges be hot-added to the core-kernel-mm as a unique
> > memory numa-node. In short, allow for the decision about what software
> > agent manages specific-purpose memory to be made at runtime.
>
> It's probably worth noting that the reason the memory lands into the
> state of being controlled by device-dax by default is that device-dax is
> nice.  It's actually willing and able to give up ownership of the memory
> when we ask.  If we added to the core-mm, we'd almost certainly not be
> able to get it back reliably.
>
> Anyway, thanks for doing these, and I really hope that the world's
> BIOSes actually use this flag.

It should be noted that the flag is necessary, but not sufficient to
route this memory range to device-dax. The BIOS must also publish ACPI
HMAT performance data for the range so the OS has a chance of knowing
*why* the memory is "reserved for a specific purpose", and delineate
the boundaries of multiple performance differentiated memory ranges
that might be combined into one shared / contiguous EFI memory
descriptor.

With no HMAT the memory will be reserved, but no dax-device will be
surfaced. Perhaps this implementation also needs a WARN_TAINT(...,
TAINT_FIRMWARE_WORKAROUND...) to scream about a BIOS that fails to
publish the required HMAT entries, or perhaps even better a command
line option to ignore the flag so that the core-mm can pick up the
memory by default?