linux-kernel - Re: [PATCH v2 4/4] iommu/arm: Add BBM Level 2 smmu feature

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Message-ID: <67fbe3f4-4fb6-4753-b34c-320b7897fd16@arm.com>
Date: Tue, 4 Mar 2025 16:02:20 +0000
From: Ryan Roberts <ryan.roberts@....com>
To: Jason Gunthorpe <jgg@...pe.ca>,
 Mikołaj Lenczewski <miko.lenczewski@....com>
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Subject: Re: [PATCH v2 4/4] iommu/arm: Add BBM Level 2 smmu feature

On 04/03/2025 14:26, Jason Gunthorpe wrote:
> On Mon, Mar 03, 2025 at 07:03:42PM +0000, Mikołaj Lenczewski wrote:
> 
>> For example, if we use BBML2 for kernel mappings, does that require us
>> to repaint all kernel mappings when disabling BBML2 on smmu attach? I
>> am not sure, but definitely something to be worked out.
> 
> No, it would be a per-mm_struct basis only if we did something like
> that
> 
> When the SMMU driver puts a SVA on top of the mm_struct it would
> disable BBML2 usage only for that mm_struct and it's contained VMAs.

I guess we would need to figure out some synchonization mechanism if disabling
BBML2 dynaically per-mm. If there was already a BBML2 operation in flight would
want to wait for it to end. But that's a problem to solve if/when it's shown to
be needed, I think.

> 
> Kernel mappings are never made into a SVA so are not effected.
> Userspace only.
> 
> Jason