Why is the ARM SMMU v1/v2 put into bypass mode on kexec?

[Will Deacon]

On Tue, Mar 19, 2024 at 06:17:39PM +0000, Robin Murphy wrote:
> In terms of the shutdown behaviour, I think it actually works out as-is. For
> the normal case we haven't touched GBPA, so we are truly returning to the
> boot-time condition; in the unexpected case where SMMUEN was already enabled
> then we'll go into an explicit GPBA abort state, but that seems a
> not-unreasonable compromise for not preserving the entire boot-time Stream
> Table etc., whose presence kind of implies it wouldn't have been bypassing
> everything anyway.
> 
> The more I look at the remaining aspect of disable_bypass for controlling
> broken-DT behaviour the more I suspect it can't actually be useful either
> way, especially not since default domains. I have no memory of what my
> original reasoning might have been, so I'm inclined to just rip that all out
> and let probe fail. I see no reason these days not to expect a broken DT to
> leads to a broken system, especially not now with DTSchema validation.

That sounds reasonable to me, although we may end up having to back it
out if we regress systems with borked firmware :(

> Then there's just the kdump warning it suppresses, of which I also have no
> idea why it's there either, but apparently that one's on you :P

I think _that_ one is because the previous (crashed) kernel won't have
torn anything down, so there could be active DMA using translations in
the SMMU. In that case, the crashkernel (which is running from some
carveout) may find the SMMU enabled, but it really can't stick it into
bypass mode because that's likely to corrupt random memory. So in that
case, we do stick it into abort before we reinitialise it and then we
disabling fault reporting altogether to avoid the log spam:

	if (is_kdump_kernel())
		enables &= ~(CR0_EVTQEN | CR0_PRIQEN)

Will