linux-kernel - Re: [PATCH v6 05/12] x86/efi: EFI soft reservation to E820 enumeration

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Date:   Thu, 10 Oct 2019 11:31:18 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
Cc:     Ingo Molnar <mingo@...hat.com>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kbuild test robot <lkp@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        ACPI Devel Maling List <linux-acpi@...r.kernel.org>
Subject: Re: [PATCH v6 05/12] x86/efi: EFI soft reservation to E820 enumeration

On Wed, Oct 9, 2019 at 11:45 PM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Thu, 10 Oct 2019 at 01:19, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > UEFI 2.8 defines an EFI_MEMORY_SP attribute bit to augment the
> > interpretation of the EFI Memory Types as "reserved for a specific
> > purpose".
> >
> > The proposed Linux behavior for specific purpose memory is that it is
> > reserved for direct-access (device-dax) by default and not available for
> > any kernel usage, not even as an OOM fallback.  Later, through udev
> > scripts or another init mechanism, these device-dax claimed ranges can
> > be reconfigured and hot-added to the available System-RAM with a unique
> > node identifier. This device-dax management scheme implements "soft" in
> > the "soft reserved" designation by allowing some or all of the
> > reservation to be recovered as typical memory. This policy can be
> > disabled at compile-time with CONFIG_EFI_SOFT_RESERVE=n, or runtime with
> > efi=nosoftreserve.
> >
> > This patch introduces 2 new concepts at once given the entanglement
> > between early boot enumeration relative to memory that can optionally be
> > reserved from the kernel page allocator by default. The new concepts
> > are:
> >
> > - E820_TYPE_SOFT_RESERVED: Upon detecting the EFI_MEMORY_SP
> >   attribute on EFI_CONVENTIONAL memory, update the E820 map with this
> >   new type. Only perform this classification if the
> >   CONFIG_EFI_SOFT_RESERVE=y policy is enabled, otherwise treat it as
> >   typical ram.
> >
> > - IORES_DESC_SOFT_RESERVED: Add a new I/O resource descriptor for
> >   a device driver to search iomem resources for application specific
> >   memory. Teach the iomem code to identify such ranges as "Soft Reserved".
> >
> > A follow-on change integrates parsing of the ACPI HMAT to identify the
> > node and sub-range boundaries of EFI_MEMORY_SP designated memory. For
> > now, just identify and reserve memory of this type.
> >
> > Cc: <x86@...nel.org>
> > Cc: Borislav Petkov <bp@...en8.de>
> > Cc: Ingo Molnar <mingo@...hat.com>
> > Cc: "H. Peter Anvin" <hpa@...or.com>
> > Cc: Darren Hart <dvhart@...radead.org>
> > Cc: Andy Shevchenko <andy@...radead.org>
> > Cc: Andy Lutomirski <luto@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Ard Biesheuvel <ard.biesheuvel@...aro.org>
> > Reported-by: kbuild test robot <lkp@...el.com>
> > Reviewed-by: Dave Hansen <dave.hansen@...ux.intel.com>
> > Signed-off-by: Dan Williams <dan.j.williams@...el.com>
>
> For the EFI changes
>
> Acked-by: Ard Biesheuvel <ard.biesheuvel@...aro.org>
>
> although I must admit I don't follow the enum add_efi_mode logic 100%

I'm open to suggestions as I'm not sure it's the best possible
organization. The do_add_efi_memmap() routine has the logic to
translate EFI to E820, but unless "add_efi_memmap" is specified on the
kernel command line the EFI memory map is ignored. For
soft-reservation support I want to reuse do_add_efi_memmap(), but
otherwise avoid any other side effects of considering the EFI map.
What I'm missing is the rationale for why "add_efi_memmap" is required
before considering the EFI memory map.

If there is a negative side effect to always using the EFI map then
the new "add_efi_mode" designation constrains it to just the
soft-reservation case.

It seems for historical reasons the full EFI memmap requires explicit opt-in:

200001eb140e x86 boot: only pick up additional EFI memmap if add_efi_memmap flag