On Wed, Sep 13, 2017 at 02:56:51PM +0200, Greg Kurz wrote: > On Wed, 13 Sep 2017 22:23:29 +1000 > David Gibson <da...@gibson.dropbear.id.au> wrote: > > [...snip...] > > > > > Also, if all PHBs are instanciated with index != -1, we're limited to > > > > > 31. > > > > > Maybe this could be the default value for the machine property > > > > > instead of > > > > > 256 then ? > > > > > > > > Actually, if we're binding it back to index, which has a hard limit, > > > > then it no longer makes sense to have it as a property and we should > > > > go back to a constant (well, it could vary by machine type version). > > > > Sorry I've taken so long to reply. > > > > Oh, don't mention it. :) > > > > But I guess that the hard limit of 31 as described in the changelog of > > > commit 357d1e3bc7d2d80e5271bc4f3ac8537e30dc8046 still holds, doesn't > > > it ? > > > > That's right. Note that that is a limit of *31* PHBs (numbered > > 0..30), not 32 PHBs numbered 0..31. > > > > Yeah I saw that. > > > > Because some guest versions (including most current distro kernels) > > > can't > > > access PCI MMIO above 64 TiB, we put all the PCI windows between 32 > > > TiB and > > > 64 TiB. This is broken into 1 TiB chunks. The first 1 TiB contains > > > the > > > PIO (64 kiB) and 32-bit MMIO (2 GiB) windows for all of the PHBs. > > > Each > > > subsequent TiB chunk contains a naturally aligned 64-bit MMIO window > > > for > > > one PHB each. > > > > > > This reduces the number of allowed PHBs (without full manual > > > configuration > > > of all the windows) from 256 to 31, but this should still be plenty in > > > practice. > > > > > > Not sure why a machine type version would have a different limit. Can > > > you think of a use case ? > > > > Well, the older machine types had a different layout. It allowed for > > more indexes, but had smaller windows, which meant certain cards (e.g. > > GPGPUs with huge BARs) didn't work properly. It also had some weird > > alignments that meant we were a bit wasteful of address space. > > > > But we can't change the location of PHB windows on migration, so we > > had to maintain that old layout for old machine types. That's why > > there's a different limit depending on machine type version. > > > > Ok, so we *just* have 2 different maximum number of PHBs: > - 256 for pseries <= 2.7 > - 31 for newer machine types
Yes, at least for now. If we ever discover that 31 PHBs *isn't*
enough, we may have to come up with yet another layout, which could
have a different limit.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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