[PATCH v2 1/3] arm64: spin-table: handle unmapped cpu-release-addrs

[Ard Biesheuvel]

On 31 July 2014 11:45, Will Deacon <will.deacon at arm.com> wrote:
> On Wed, Jul 30, 2014 at 08:17:02PM +0100, Ard Biesheuvel wrote:
>> ]On 30 July 2014 13:30, Will Deacon <will.deacon at arm.com> wrote:
>> > On Wed, Jul 30, 2014 at 11:59:02AM +0100, Ard Biesheuvel wrote:
>> >> From: Mark Rutland <mark.rutland at arm.com>
>> >>
>> >> In certain cases the cpu-release-addr of a CPU may not fall in the
>> >> linear mapping (e.g. when the kernel is loaded above this address due to
>> >> the presence of other images in memory). This is problematic for the
>> >> spin-table code as it assumes that it can trivially convert a
>> >> cpu-release-addr to a valid VA in the linear map.
>> >>
>> >> This patch modifies the spin-table code to use a temporary cached
>> >> mapping to write to a given cpu-release-addr, enabling us to support
>> >> addresses regardless of whether they are covered by the linear mapping.
>> >>
>> >> Signed-off-by: Mark Rutland <mark.rutland at arm.com>
>> >> Tested-by: Mark Salter <msalter at redhat.com>
>> >> [ardb: added (__force void *) cast]
>> >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel at linaro.org>
>> >> ---
>> >>  arch/arm64/kernel/smp_spin_table.c | 22 +++++++++++++++++-----
>> >>  1 file changed, 17 insertions(+), 5 deletions(-)
>> >
>> > I'm nervous about this. What if the spin table sits in the same physical 64k
>> > frame as a read-sensitive device and we're running with 64k pages?
>> >
>>
>> Actually, booting.txt requires cpu-release-addr to point to a
>> /memreserve/d part of memory, which implies DRAM (or you wouldn't have
>> to memreserve it)
>> That means it should always be covered by the linear mapping, unless
>> it is located before Image in DRAM, which is the case addressed by
>> this patch.
>
> But if it's located before before the Image in DRAM and isn't covered by
> the linear mapping, then surely the /memreserve/ is pointless too? In which

No, it isn't. The existence of a linear mapping and where exactly it
starts is an implementation detail of arm64 linux, whereas marking
some regions of DRAM as containing firmware bits that should be left
alone by the OS has a purpose in general. Also, with TEXT_OFFSET
likely being changed in the future, the start of the linear mapping
may change as well.

> case, this looks like we're simply trying to cater for platforms that aren't
> following booting.txt (which may need updating if we need to handle this).
>

booting.txt is not clear about the purpose of TEXT_OFFSET or whether
the area below it should be kept vacant. So APM Mustang, for instance,
keeps their holding pen there, which is entirely legal by the current
wording of booting.txt. And it is memreserve'd, so the linear mapping
will cover it but the memory will not be touched other than to bring
up the secondaries.

The point I was trying to make is that booting.txt seems to suggest
(but it should be clarified) that cpu-release-addr must always reside
on DRAM, in which case it is highly unlikely that some peripheral mem
region with I/O semantics shares its 64k page frame.

-- 
Ard.