Re: [PATCH RFC v2 13/27] arm64: mte: Make tag storage depend on ARCH_KEEP_MEMBLOCK

[David Hildenbrand]

On 27.11.23 16:04, Alexandru Elisei wrote:
> Hi,
>
> On Fri, Nov 24, 2023 at 08:51:38PM +0100, David Hildenbrand wrote:
> > On 19.11.23 17:57, Alexandru Elisei wrote:
> > > Tag storage memory requires that the tag storage pages used for data are
> > > always migratable when they need to be repurposed to store tags.
> > >
> > > If ARCH_KEEP_MEMBLOCK is enabled, kexec will scan all non-reserved
> > > memblocks to find a suitable location for copying the kernel image. The
> > > kernel image, once loaded, cannot be moved to another location in physical
> > > memory. The initialization code for the tag storage reserves the memblocks
> > > for the tag storage pages, which means kexec will not use them, and the tag
> > > storage pages can be migrated at any time, which is the desired behaviour.
> > >
> > > However, if ARCH_KEEP_MEMBLOCK is not selected, kexec will not skip a
> > > region unless the memory resource has the IORESOURCE_SYSRAM_DRIVER_MANAGED
> > > flag, which isn't currently set by the tag storage initialization code.
> > >
> > > Make ARM64_MTE_TAG_STORAGE depend on ARCH_KEEP_MEMBLOCK to make it explicit
> > > that that the Kconfig option required for it to work correctly.
> > >
> > > Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> > > ---
> > >    arch/arm64/Kconfig | 1 +
> > >    1 file changed, 1 insertion(+)
> > >
> > > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > > index 047487046e8f..efa5b7958169 100644
> > > --- a/arch/arm64/Kconfig
> > > +++ b/arch/arm64/Kconfig
> > > @@ -2065,6 +2065,7 @@ config ARM64_MTE
> > >    if ARM64_MTE
> > >    config ARM64_MTE_TAG_STORAGE
> > >    	bool "Dynamic MTE tag storage management"
> > > +	depends on ARCH_KEEP_MEMBLOCK
> > >    	select CONFIG_CMA
> > >    	help
> > >    	  Adds support for dynamic management of the memory used by the hardware
> >
> > Doesn't arm64 select that unconditionally? Why is this required then?
>
> I've added this patch to make the dependancy explicit. If, in the future, arm64
> stops selecting ARCH_KEEP_MEMBLOCK, I thinkg it would be very easy to miss the
> fact that tag storage depends on it. So this patch is not required per-se, it's
> there to document the dependancy.

I see. Could you add some static_assert / BUILD_BUG_ON instead?

I suspect there are plenty other (undocumented) reasons why 
ARCH_KEEP_MEMBLOCK has to be enabled for now, and none sets 
ARCH_KEEP_MEMBLOCK, I suspect/

--
Cheers,

David / dhildenb