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Date:   Tue, 28 Nov 2023 18:05:20 +0100
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Subject: Re: [PATCH RFC v2 13/27] arm64: mte: Make tag storage depend on
 ARCH_KEEP_MEMBLOCK
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To:     Alexandru Elisei <alexandru.elisei@arm.com>
Cc:     catalin.marinas@arm.com, will@kernel.org, oliver.upton@linux.dev,
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References: <20231119165721.9849-1-alexandru.elisei@arm.com>
 <20231119165721.9849-14-alexandru.elisei@arm.com>
 <91c5d2e2-57b1-4172-88e0-cd07a8d85af4@redhat.com> <ZWSwArYMN1LuyGfO@raptor>
From:   David Hildenbrand <david@redhat.com>
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In-Reply-To: <ZWSwArYMN1LuyGfO@raptor>
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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@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.

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