Re: [PATCH v3 0/3] arm64: mm: Do not defer reserve_crashkernel()

[Catalin Marinas]

On Mon, Apr 11, 2022 at 05:24:52PM +0800, Kefeng Wang wrote:
> Commit 031495635b46 ("arm64: Do not defer reserve_crashkernel() for
> platforms with no DMA memory zones"), this lets the kernel benifit
> due to BLOCK_MAPPINGS, we could do more if ZONE_DMA and ZONE_DMA32
> enabled.
> 
> 1) Don't defer reserve_crashkernel() if only ZONE_DMA32
> 2) Don't defer reserve_crashkernel() if ZONE_DMA with dma_force_32bit
>    kernel parameter(newly added)

I'm not really keen on a new kernel parameter for this. But even with
such parameter, there is another series that allows crashkernel
reservations above ZONE_DMA32, so that would also need
NO_BLOCK_MAPPINGS, at least initially. I think there was a proposal to
do the high reservation first and only defer the low one in ZONE_DMA but
suggested we get the reservations sorted first and look at optimisations
later.

If hardware is so bad with page mappings, I think we need to look at
different ways to enable the block mappings, e.g. some safe break
before make change of the mappings or maybe switching to another TTBR1
during boot.

Does FEAT_BBM level 2 allow us to change the block size without a break
before make? I think that can still trigger a TLB conflict abort, maybe
we can trap it and invalidate the TLBs (the conflict should be on the
linear map not where the kernel image is mapped).

-- 
Catalin