Mauricio Faria de Oliveira <mauri...@linux.vnet.ibm.com> writes: > Hi Michal and Michael, > > On 02/15/2018 05:13 AM, Michal Suchánek wrote: >>> From: Michael Ellerman<m...@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