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From:   Ard Biesheuvel <ardb@kernel.org>
Date:   Wed, 9 Sep 2020 13:46:59 +0300
Message-ID: <CAMj1kXGkhJqMFggO7reedKkuQh2vmNeac5pXirEcN+XvMX==gA@mail.gmail.com>
Subject: Re: [PATCH 1/1] efi/libstub: DRAM base calculation
To:     Maxim Uvarov <maxim.uvarov@linaro.org>
Cc:     Heinrich Schuchardt <xypron.glpk@gmx.de>,
        Atish Patra <atish.patra@wdc.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Ingo Molnar <mingo@kernel.org>,
        Arvind Sankar <nivedita@alum.mit.edu>,
        linux-efi <linux-efi@vger.kernel.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Jens Wiklander <jens.wiklander@linaro.org>,
        =?UTF-8?Q?Fran=C3=A7ois_Ozog?= <francois.ozog@linaro.org>
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On Wed, 9 Sep 2020 at 13:44, Maxim Uvarov <maxim.uvarov@linaro.org> wrote:
>
> On Wed, 9 Sep 2020 at 11:17, Ard Biesheuvel <ardb@kernel.org> wrote:
> >
> > (+ Atish, Palmer)
> >
> > On Fri, 4 Sep 2020 at 18:50, Heinrich Schuchardt <xypron.glpk@gmx.de> wrote:
> > >
> > > In the memory map the regions with the lowest addresses may be of type
> > > EFI_RESERVED_TYPE. The reserved areas may be discontinuous relative to the
> > > rest of the memory. So for calculating the maximum loading address for the
> > > device tree and the initial ramdisk image these reserved areas should not
> > > be taken into account.
> > >
> > > Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
> > > ---
> > >  drivers/firmware/efi/libstub/efi-stub.c | 3 ++-
> > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/drivers/firmware/efi/libstub/efi-stub.c b/drivers/firmware/efi/libstub/efi-stub.c
> > > index c2484bf75c5d..13058ac75765 100644
> > > --- a/drivers/firmware/efi/libstub/efi-stub.c
> > > +++ b/drivers/firmware/efi/libstub/efi-stub.c
> > > @@ -106,7 +106,8 @@ static unsigned long get_dram_base(void)
> > >         map.map_end = map.map + map_size;
> > >
> > >         for_each_efi_memory_desc_in_map(&map, md) {
> > > -               if (md->attribute & EFI_MEMORY_WB) {
> > > +               if (md->attribute & EFI_MEMORY_WB &&
> > > +                   md->type != EFI_RESERVED_TYPE) {
> > >                         if (membase > md->phys_addr)
> > >                                 membase = md->phys_addr;
> > >                 }
> > > --
> > > 2.28.0
> > >
> >
> > This is not the right fix - on RPi2, for instance, which has some
> > reserved memory at the base of DRAM, this change will result in the
> > first 16 MB of memory to be wasted.
> >
> In the EFI memmap provided to the kernel efi stub it will be 2
> regions. First is EFI_RESERVED and second is EFI_CONVENTIONAL_MEMORY.
> Even if they follow each other.
> And for_each_efi_memory_desc_in_map will just return the second one.
> Do not see where the problem is here.
>

The base of DRAM will no longer start at a 16 MB aligned address on
RPi, and so it will round up to the next 16 MB, wasting the space in
between.

> > What I would prefer to do is get rid of get_dram_base() entirely -
> > arm64 does not use its return value in the first place, and for ARM,
> > the only reason we need it is so that we can place the uncompressed
> > kernel image as low in memory as possible, and there are probably
> > better ways to do that. RISC-V just started using it too, but only
> > passes it from handle_kernel_image() to efi_relocate_kernel(), and
> > afaict, passing 0x0 there instead would not cause any problems.
>
> For prior 5.8 kernels there was limitation for maximum address to
> unpack the kernel. As I understand that was copy-pasted from x86 code,
> and now is missing in 5.9.

What code are you referring to here?

> That is why the suggestion was to point
> dram_base to the region where it's possible to allocate. I.e. I assume
> that
> patch was created not to the latest kernel. Removing the upper
> allocation limit should work here.
>

As I pointed out, this will regress other platforms.