[PATCH 2/3] rfi-flush: Make it possible to call setup_rfi_flush() again

[Michael Ellerman]

Mauricio Faria de Oliveira <mauricfo at linux.vnet.ibm.com> writes:

> Hi Michal and Michael,
>
> On 02/15/2018 05:13 AM, Michal Suchánek wrote:
>>> From: Michael Ellerman<mpe at ellerman.id.au>
>>>
>>> For PowerVM migration we want to be able to call setup_rfi_flush()
>>> again after we've migrated the partition.
>>>
>>> To support that we need to check that we're not trying to allocate the
>>> fallback flush area after memblock has gone away. If so we just fail,
>>> we don't support migrating from a patched to an unpatched machine. Or
>>> we do support it, but there will be no RFI flush enabled on the
>>> destination.
>>>
>> This sounds bad to me. Either we support RFI flush or we don't.
>> 
>> If we do the fallback area should be allocated at boot so it is always
>> available. [snip]
>
> I think the problem with this is the size of the fallback area might
> have to be different between the origin and destination systems, say,
> a larger L1 data cache at the destination.
>
> In that case, the original size might not be enough to fully flush
> the L1 data cache.
>
> Michael, is that the reason it is done that way?  I thought of that,
> but don't know for sure.

No, supporting different cache sizes is a good idea though :)

I did it the way I did because otherwise we waste memory on every system
on earth just to support a use case that we don't actually intend for
anyone to ever use - ie. migrating from a patched machine to an
unpatched machine.

In fact without further checks we'd be allocating the fallback area on
powernv machines which don't even support LPM.

So that just seemed a bit gross.

I think I'm inclined to leave it the way it is, unless you feel strongly
about it Michal?

cheers