Re: [PATCH 1/1] efi/libstub: DRAM base calculation

[Maxim Uvarov]

On Wed, 9 Sep 2020 at 13:47, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
>
> On Wed, 9 Sep 2020 at 13:44, Maxim Uvarov <maxim.uvarov@xxxxxxxxxx> wrote:
> >
> > On Wed, 9 Sep 2020 at 11:17, Ard Biesheuvel <ardb@xxxxxxxxxx> wrote:
> > >
> > > (+ Atish, Palmer)
> > >
> > > On Fri, 4 Sep 2020 at 18:50, Heinrich Schuchardt <xypron.glpk@xxxxxx> 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@xxxxxx>
> > > > ---
> > > >  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.
>

Ok.

> > > 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?
>
to this commit:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/drivers/firmware/efi/libstub/arm32-stub.c?h=v5.9-rc4&id=d0f9ca9be11f25ef4151195eab7ea36d136084f6

> > 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.