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

[Ard Biesheuvel]

On 30 July 2014 14:49, Mark Rutland <mark.rutland at arm.com> wrote:
> On Wed, Jul 30, 2014 at 01:42:58PM +0100, Will Deacon wrote:
>> On Wed, Jul 30, 2014 at 01:30:29PM +0100, Mark Rutland wrote:
>> > On Wed, Jul 30, 2014 at 01:00:40PM +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?
>> > > >
>> > >
>> > > I see what you mean. This is potentially hairy, as EFI already
>> > > ioremap_cache()s everything known to it as normal DRAM, so using plain
>> > > ioremap() here if pfn_valid() returns false for cpu-release-addr's PFN
>> > > may still result in mappings with different attributes for the same
>> > > region. So how should we decide whether to call ioremap() or
>> > > ioremap_cache() in this case?
>> >
>> > If we're careful about handling mismatched attributes we might be able
>> > to get away with always using a device mapping.
>>
>> Even then, I think ioremap hits a WARN_ON if pfn_valid.
>
> Ok, that's that idea dead then.
>
>> > I'll need to have a think about that, I'm not sure on the architected
>> > cache behaviour in such a case.
>>
>> Of we just skip the cache flush if !pfn_valid.
>
> I don't think that's always safe given Ard's comment that the EFI code
> will possibly have a mapping covering the region created by
> ioremap_cache.
>
> Ard, what exactly does the EFI code map with ioremap_cache, and why?
>

Actually, after re-reading the spec and the code, perhaps this is not an issue.
The EFI __init code calls ioremap_cache() for all regions described by
the UEFI memory map as requiring a virtual mapping
(EFI_MEMORY_RUNTIME): this is primarily runtime services code and data
regions and perhaps some I/O mappings for flash or other peripherals
that UEFI owns and needs to access during Runtime Services calls.

Mark Salter mentioned that APM Mustang's spin table lives in an
EFI_RESERVED_TYPE region, which presumably would not have the
EFI_MEMORY_RUNTIME attribute set, as it has nothing to do with the
UEFI Runtime Services. This means that no cached mapping should
already exist for that region.

-- 
Ard.