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

[Alexandru Elisei]

Hi,

On Tue, Nov 28, 2023 at 06:05:20PM +0100, David Hildenbrand wrote:
> 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 can do that, sure.

Thanks,
Alex

> 
> 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
>