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

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Date:   Sat, 8 Jun 2019 09:20:13 +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, 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.

Perhaps a efi=xxx command line option would be in order to influence
the builtin default, but it can be a followup patch independent of
this series.