Re: [PATCH] Revert "MdePkg/ProcessorBind.h AARCH64: limit MAX_ADDRESS to 48 bits"

[Ard Biesheuvel <ard.biesheuvel@linaro.org>]

On Fri, 7 Dec 2018 at 11:41, Laszlo Ersek <lersek@redhat.com> wrote:
>
> On 12/06/18 22:37, Ard Biesheuvel wrote:
> > This reverts commit 82379bf6603274e81604d5a6f6bb14bdde616286.
> >
> > On AArch64, we can only use 48 address bits while running in UEFI,
> > while the GCD and UEFI memory maps may describe up to 52 bits of
> > physical address space. For this reason, MAX_ADDRESS was reduced
> > to 48 bits, to ensure that the firmware does not inadvertently
> > attempt to allocate memory that we cannot access.
> >
> > However, MAX_ADDRESS is used in runtime drivers as well, and
> > runtime drivers may deal with kernel virtual addresses, which have
> > bits [63:48] set. In fact, the OS may be running with 64 KB pages
> > and pass addresses into the runtime services that use up to 52
> > bits of address space, either with the top bits set or cleared,
> > even if the physical address space does not extend beyond 48 bits.
> >
> > In summary, changing MAX_ADDRESS is a mistake, and needs to be
> > reverted.
> >
> > Cc: Leif Lindholm <leif.lindholm@linaro.org>
> > Cc: Liming Gao <liming.gao@intel.com>
> > Cc: Laszlo Ersek <lersek@redhat.com>
> > Cc: Eric Auger <eric.auger@redhat.com>
> > Cc: Andrew Jones <drjones@redhat.com>
> > Cc: Philippe Mathieu-Daude <philmd@redhat.com>
> > Contributed-under: TianoCore Contribution Agreement 1.1
> > Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
> > ---
> >  MdePkg/Include/AArch64/ProcessorBind.h | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> >
> > diff --git a/MdePkg/Include/AArch64/ProcessorBind.h b/MdePkg/Include/AArch64/ProcessorBind.h
> > index dad75df1c579..968c18f915ae 100644
> > --- a/MdePkg/Include/AArch64/ProcessorBind.h
> > +++ b/MdePkg/Include/AArch64/ProcessorBind.h
> > @@ -138,9 +138,9 @@ typedef INT64   INTN;
> >  #define MAX_2_BITS  0xC000000000000000ULL
> >
> >  ///
> > -/// Maximum legal AARCH64  address (48 bits for 4 KB page size)
> > +/// Maximum legal AARCH64  address
> >  ///
> > -#define MAX_ADDRESS   0xFFFFFFFFFFFFULL
> > +#define MAX_ADDRESS   0xFFFFFFFFFFFFFFFFULL
> >
> >  ///
> >  /// Maximum legal AArch64 INTN and UINTN values.
> >
>
> I was worried the patch could regress some things, but unfortunately, I
> couldn't name any specific area of concern. Sorry about that.
>

Sometimes a hunch should be taken more seriously, I suppose :-)

> For this change:
>
> Reviewed-by: Laszlo Ersek <lersek@redhat.com>
>

Thanks

> Topic change: the patch that's being reverted was originally posted as:
>
>   [edk2] [PATCH v3 05/16]
>   MdePkg/ProcessorBind.h AARCH64: limit MAX_ADDRESS to 48 bits
>
> in the series
>
>   [edk2] [PATCH v3 00/16]
>   [Arm|ArmVirt|MdePkg|Embedded]Pkg: lift 40-bit IPA space limit
>
> In further patches of that series, we depended on the lowered limit of
> MAX_ADDRESS. Given that MAX_ADDRESS is being raised back to its original
> value, I think those dependent locations should be re-checked.
>
> For example, in
>
>   [edk2] [PATCH v3 08/16]
>   ArmPkg/ArmMmuLib: take the CPU supported maximum PA space into account
>
> (commit e36b243c7178), we added
>
>   //
>   // Limit the virtual address space to what we can actually use: UEFI
>   // mandates a 1:1 mapping, so no point in making the virtual address
>   // space larger than the physical address space. We also have to take
>   // into account the architectural limitations that result from UEFI's
>   // use of 4 KB pages.
>   //
>   MaxAddress = MIN (LShiftU64 (1ULL, ArmGetPhysicalAddressBits ()) - 1,
>                     MAX_ADDRESS);
>
> Presumably, we should now replace MAX_ADDRESS with 0xFFFFFFFFFFFFULL.
>
> (I'm unsure if other modules updated by the rest of the patches are
> affected -- I tried to grep them for MAX_ADDRESS, and I couldn't find
> any (obvious) matches.)
>

I am looking into an alternative approach, involving the introduction
of MAX_ALLOC_ADDRESS, and updating the page allocator to honour it.
The MMU library should be updated to refer to it as well, thanks for
reminding me.