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

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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@xxxxxxx> wrote:
> > > > On Wed, Jul 30, 2014 at 11:59:02AM +0100, Ard Biesheuvel wrote:
> > > >> From: Mark Rutland <mark.rutland@xxxxxxx>
> > > >>
> > > >> 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@xxxxxxx>
> > > >> Tested-by: Mark Salter <msalter@xxxxxxxxxx>
> > > >> [ardb: added (__force void *) cast]
> > > >> Signed-off-by: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
> > > >> ---
> > > >>  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?

Cheers,
Mark.
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