linux-kernel - Re: [PATCH v2 4/8] x86, efi: Reserve UEFI 2.8 Specific Purpose Memory for dax

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Message-ID: <CAKv+Gu83QB6x8=LCaAcR0S65WELC-Y+Voxw6LzaVh4FSV3bxYA@mail.gmail.com>
Date:   Fri, 31 May 2019 17:30:32 +0200
From:   Ard Biesheuvel <ard.biesheuvel@...aro.org>
To:     Dan Williams <dan.j.williams@...el.com>
Cc:     Mike Rapoport <rppt@...ux.ibm.com>,
        linux-efi <linux-efi@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Borislav Petkov <bp@...en8.de>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Darren Hart <dvhart@...radead.org>,
        Andy Shevchenko <andy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        kbuild test robot <lkp@...el.com>,
        Vishal L Verma <vishal.l.verma@...el.com>,
        Linux-MM <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH v2 4/8] x86, efi: Reserve UEFI 2.8 Specific Purpose Memory
 for dax

On Fri, 31 May 2019 at 17:28, Dan Williams <dan.j.williams@...el.com> wrote:
>
> On Fri, May 31, 2019 at 1:30 AM Ard Biesheuvel
> <ard.biesheuvel@...aro.org> wrote:
> >
> > (cc Mike for memblock)
> >
> > On Fri, 31 May 2019 at 01:13, 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 special
> > > 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 patch introduces 3 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_SPECIFIC: 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_SPECIFIC_DAX=y policy is
> > >   enabled, otherwise treat it as typical ram.
> > >
> >
> > OK, so now we have 'special purpose', 'specific' and 'app specific'
> > [below]. Do they all mean the same thing?
>
> I struggled with separating the raw-EFI-type name from the name of the
> Linux specific policy. Since the reservation behavior is optional I
> was thinking there should be a distinct Linux kernel name for that
> policy. I did try to go back and change all occurrences of "special"
> to "specific" from the RFC to this v2, but seems I missed one.
>

OK

> >
> > > - IORES_DESC_APPLICATION_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 "Application
> > >   Reserved".
> > >
> > > - MEMBLOCK_APP_SPECIFIC: Given the memory ranges can fallback to the
> > >   traditional System RAM pool the expectation is that they will have
> > >   typical SRAT entries. In order to support a policy of device-dax by
> > >   default with the option to hotplug later, the numa initialization code
> > >   is taught to avoid marking online MEMBLOCK_APP_SPECIFIC regions.
> > >
> >
> > Can we move the generic memblock changes into a separate patch please?
>
> Yeah, that can move to a lead-in patch.
>
> [..]
> > > diff --git a/include/linux/efi.h b/include/linux/efi.h
> > > index 91368f5ce114..b57b123cbdf9 100644
> > > --- a/include/linux/efi.h
> > > +++ b/include/linux/efi.h
> > > @@ -129,6 +129,19 @@ typedef struct {
> > >         u64 attribute;
> > >  } efi_memory_desc_t;
> > >
> > > +#ifdef CONFIG_EFI_SPECIFIC_DAX
> > > +static inline bool is_efi_dax(efi_memory_desc_t *md)
> > > +{
> > > +       return md->type == EFI_CONVENTIONAL_MEMORY
> > > +               && (md->attribute & EFI_MEMORY_SP);
> > > +}
> > > +#else
> > > +static inline bool is_efi_dax(efi_memory_desc_t *md)
> > > +{
> > > +       return false;
> > > +}
> > > +#endif
> > > +
> > >  typedef struct {
> > >         efi_guid_t guid;
> > >         u32 headersize;
> >
> > I'd prefer it if we could avoid this DAX policy distinction leaking
> > into the EFI layer.
> >
> > IOW, I am fine with having a 'is_efi_sp_memory()' helper here, but
> > whether that is DAX memory or not should be decided in the DAX layer.
>
> Ok, how about is_efi_sp_ram()? Since EFI_MEMORY_SP might be applied to
> things that aren't EFI_CONVENTIONAL_MEMORY.

Yes, that is fine. As long as the #ifdef lives in the DAX code and not here.