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

[Ard Biesheuvel]

On Fri, 11 Oct 2019 at 16:35, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
>
> On Thu, Oct 10, 2019 at 10:52 PM Ard Biesheuvel
> <ard.biesheuvel@xxxxxxxxxx> wrote:
> >
> > On Fri, 11 Oct 2019 at 04:39, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > >
> > > On Thu, Oct 10, 2019 at 11:41 AM Ard Biesheuvel
> > > <ard.biesheuvel@xxxxxxxxxx> wrote:
> > > >
> > > > On Thu, 10 Oct 2019 at 20:31, Dan Williams <dan.j.williams@xxxxxxxxx> wrote:
> > > > >
> > > > > On Wed, Oct 9, 2019 at 11:45 PM Ard Biesheuvel
> > > > > <ard.biesheuvel@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > On Thu, 10 Oct 2019 at 01:19, Dan Williams <dan.j.williams@xxxxxxxxx> 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@xxxxxxxxxx>
> > > > > > > Cc: Borislav Petkov <bp@xxxxxxxxx>
> > > > > > > Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> > > > > > > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> > > > > > > Cc: Darren Hart <dvhart@xxxxxxxxxxxxx>
> > > > > > > Cc: Andy Shevchenko <andy@xxxxxxxxxxxxx>
> > > > > > > Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> > > > > > > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > > > > > > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > > > > > > Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > > > > > Reported-by: kbuild test robot <lkp@xxxxxxxxx>
> > > > > > > Reviewed-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> > > > > > > Signed-off-by: Dan Williams <dan.j.williams@xxxxxxxxx>
> > > > > >
> > > > > > For the EFI changes
> > > > > >
> > > > > > Acked-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > > > >
> > > > > > 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.
> > > > >
> > > >
> > > > Could we make the presence of any EFI_MEMORY_SP regions imply
> > > > add_efi_memmap? That way, it is guaranteed that we don't regress
> > > > existing systems, while establishing clear and unambiguous semantics
> > > > for new systems that rely on these changes in order to be able to use
> > > > the special purpose memory as intended.
> > >
> > > In fact that's how it works. EFI_MEMORY_SP is unconditionally added.
> > > Other EFI memory types are optionally added with the add_efi_memmap
> > > option.
> >
> > That is not what I meant.
> >
> > Why not behave as if 'add_efi_memmap' was passed if any EFI_MEMORY_SP
> > regions exist?
>
> Hmm, ok, on the assumption that any platform that is modern enough to
> specify EFI_MEMORY_SP likely does not need the opt-in?

Indeed.

> I can get on
> board with that. It's also simple enough to undo if it causes problems
> in practice.

Yes, and we'll only have one code path to reason about here.