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Date:   Mon, 27 Nov 2023 15:04:34 +0000
From:   Alexandru Elisei <alexandru.elisei@arm.com>
To:     David Hildenbrand <david@redhat.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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Subject: Re: [PATCH RFC v2 13/27] arm64: mte: Make tag storage depend on
 ARCH_KEEP_MEMBLOCK
Message-ID: <ZWSwArYMN1LuyGfO@raptor>
References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-14-alexandru.elisei@arm.com>
 <91c5d2e2-57b1-4172-88e0-cd07a8d85af4@redhat.com>
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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@arm.com>
> > ---
> >   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.

Thanks,
Alex

> 
> -- 
> Cheers,
> 
> David / dhildenb
>