Re: [PATCH v2] swiotlb: avoid potential left shift overflow

[Chao Gao]

On Fri, Aug 19, 2022 at 06:44:05AM -0700, Dongli Zhang wrote:
>I also encountered this when sending out another version of the 64-bit swiotlb.
>
>https://lore.kernel.org/all/20220609005553.30954-8-dongli.zhang@xxxxxxxxxx/
>
>Unfortunately, I could not find an environment (e.g., powerpc) to allocate more
>than 4G until swiotlb supports 64-bit.
>
>Although xen supports 64-bit, but the hypervisor side limits the max to < 4G.

Sorry. I didn't notice your series before. I agree that the overflow
isn't an issue if swiotlb size cannot be larger than 4GB. That's why
I said the overflow is a potential issue.

In an internal effort to measure the impact of swiotlb size to IO
performance of confidential VM (e.g., TDX VM), we simply added
SWIOTLB_ANY to the default io_tlb_mem to lift the restriction on swiotlb
size. Then we hit this issue and worked out this fix. I posted this
fix because I think the fix by itself is helpful because it removes the
implicit dependency of the left-shift in slot_addr() on swiotlb size and
then someone trying to lift the size limitation won't hit the same issue.