Re: [PATCH v5] arm64: mm: fix linear mem mapping access performance degradation

[Will Deacon <will@kernel.org>]

On Sun, Jul 10, 2022 at 11:33:02PM +0800, guanghui.fgh wrote:
> In short, this path work:
> 
> 1.Before doing work for rebuiling crashkernel mem, the pgd is swapper_pg_dir
> in [[[ttbr1]]]
> 
> 2.Change the [[[ttbr0]]]to use idmap_pg_dir pgd
> 
> 3.The [[[idmap_cpu_replace_ttbr1_with_flush_tlb]]] are mapped [[[only]]]
> with idmap_pg_dir mapping in [[[ttbr0]]]
> 
> 4.The [[[idmap_cpu_replace_ttbr1_with_flush_tlb]]] will flush tlb all,
> switch [[[ttbr1]]] to use init_pg_dir pgd(and flush tlb all again).
> There is no tlb conflict to swapper_pg_dir.
> There is no tlb cache for swapper_pg_dir.
> 
> 5.Woring with init_pg_dir pgd to access swapper_pg_dir pagetable with fix
> mapping. And modify crashkernel mapping in the swapper_pg_dir without any
> tlb conflict and flush.
> 
> 6.When finishing the work, switch ttbr1 pgd to the origin swapper_pg_dir
> with cpu_replace_ttbr1 function(similar to the above).

I do not think that this complexity is justified. As I have stated on
numerous occasions already, I would prefer that we leave the crashkernel
mapped when rodata is not "full". That fixes your performance issue and
matches what we do for module code, so I do not see a security argument
against it.

I do not plan to merge this patch as-is.

Thanks,

Will