linux-kernel - Re: [RFC PATCH 0/3] arm64: support FEAT_BBM level 2 and large block mapping when rodata=full

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Message-ID: <20241210113151.GC14735@willie-the-truck>
Date: Tue, 10 Dec 2024 11:31:52 +0000
From: Will Deacon <will@...nel.org>
To: Yang Shi <yang@...amperecomputing.com>
Cc: catalin.marinas@....com, cl@...two.org, scott@...amperecomputing.com,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 0/3] arm64: support FEAT_BBM level 2 and large block
 mapping when rodata=full

On Mon, Nov 18, 2024 at 10:16:07AM -0800, Yang Shi wrote:
> 
> When rodata=full kernel linear mapping is mapped by PTE due to arm's
> break-before-make rule.
> 
> This resulted in a couple of problems:
>   - performance degradation
>   - more TLB pressure
>   - memory waste for kernel page table
> 
> There are some workarounds to mitigate the problems, for example, using
> rodata=on, but this compromises the security measurement.
> 
> With FEAT_BBM level 2 support, splitting large block page table to
> smaller ones doesn't need to make the page table entry invalid anymore.
> This allows kernel split large block mapping on the fly.

I think you can still get TLB conflict aborts in this case, so this
doesn't work. Hopefully the architecture can strengthen this in the
future to give you what you need.

Will