[PATCH 2/4] iommu/arm-smmu-v3: Document MMU-700 erratum 2812531

[Robin Murphy]

On 2023-05-10 19:31, Nicolin Chen wrote:
> On Wed, May 10, 2023 at 11:12:05AM -0700, Nicolin Chen wrote:
>> On Wed, May 10, 2023 at 04:38:44PM +0100, Robin Murphy wrote:
>>
>>> To work around MMU-700 erratum 2812531 we need to ensure that certain
>>> sequences of commands cannot be issued without an intervening sync. In
>>> practice this falls out of our current command-batching machinery
>>> anyway - each batch only contains a single type of invalidation command,
>>
>> Hmm. This doesn't apply to the user cache invalidation solution
>> in my v2. A user cmdq could possibly mix different commands in
>> a single batch if the driver isn't aware of such an errata. So,
>> I think I'd need some twist when the host has a FORCE_SYNC flag
>> in my v3.
> 
> Just found that you unset the NESTING feature flag in PATCH-4
> for this errata too. So, the solution in my v2 should be safe.

For the short term at least - we will still need to come up with a more 
practical active mitigation sooner rather than later, since I imagine 
people are likely to be a bit miffed if nesting support lands in distros 
but still refuses to play at all on the newest shiniest hardware. 
Unfortunately all current versions of MMU-700 are affected (hence the 
unconditional nature of this patch), and some of those are going to 
found in production SoCs.

Thanks,
Robin.