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

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Message-ID: <CAPcyv4hB7EbxkcDGc1j2vXwFcX5rHOYtRZcRa7Q36CVrAk1w+g@mail.gmail.com>
Date:   Fri, 21 Jun 2019 13:06:38 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Ard Biesheuvel <ard.biesheuvel@...aro.org>
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 Sat, Jun 8, 2019 at 12:20 AM Ard Biesheuvel
<ard.biesheuvel@...aro.org> wrote:
>
> On Fri, 7 Jun 2019 at 19:34, Dan Williams <dan.j.williams@...el.com> wrote:
> >
> > On Fri, Jun 7, 2019 at 8:23 AM Dan Williams <dan.j.williams@...el.com> wrote:
> > >
> > > On Fri, Jun 7, 2019 at 5:29 AM Ard Biesheuvel <ard.biesheuvel@...aro.org> wrote:
> > [..]
> > > > > #ifdef CONFIG_EFI_APPLICATION_RESERVED
> > > > > static inline bool is_efi_application_reserved(efi_memory_desc_t *md)
> > > > > {
> > > > >         return md->type == EFI_CONVENTIONAL_MEMORY
> > > > >                 && (md->attribute & EFI_MEMORY_SP);
> > > > > }
> > > > > #else
> > > > > static inline bool is_efi_application_reserved(efi_memory_desc_t *md)
> > > > > {
> > > > >         return false;
> > > > > }
> > > > > #endif
> > > >
> > > > I think this policy decision should not live inside the EFI subsystem.
> > > > EFI just gives you the memory map, and mangling that information
> > > > depending on whether you think a certain memory attribute should be
> > > > ignored is the job of the MM subsystem.
> > >
> > > The problem is that we don't have an mm subsystem at the time a
> > > decision needs to be made. The reservation policy needs to be deployed
> > > before even memblock has been initialized in order to keep kernel
> > > allocations out of the reservation. I agree with the sentiment I just
> > > don't see how to practically achieve an optional "System RAM" vs
> > > "Application Reserved" routing decision without an early (before
> > > e820__memblock_setup()) conditional branch.
> >
> > I can at least move it out of include/linux/efi.h and move it to
> > arch/x86/include/asm/efi.h since it is an x86 specific policy decision
> > / implementation for now.
>
> No, that doesn't make sense to me. If it must live in the EFI
> subsystem, I'd prefer it to be in the core code, not in x86 specific
> code, since there is nothing x86 specific about it.

The decision on whether / if to take any action on this hint is
implementation specific, so I argue it does not belong in the EFI
core. The spec does not mandate any action as it's just a hint.
Instead x86 is making a policy decision in how it translates it to the
x86-specific E820 representation. So, I as I go to release v3 of this
patch set I do not see an argument to move the
is_efi_application_reserved() definition out of
arch/x86/include/asm/efi.h it's 100% tied to the e820 translation.

Now, if some other EFI supporting architecture wanted to follow the
x86 policy we could move it it to a shared location, but that's
something for a follow-on patch set.