Petersson, Mats
2006-May-03 13:23 UTC
RE: [Xen-users] How does Xen present processors to DomUs?
> -----Original Message----- > From: xen-users-bounces@lists.xensource.com > [mailto:xen-users-bounces@lists.xensource.com] On Behalf Of > Gregory Cymbalski > Sent: 03 May 2006 14:08 > To: xen-users@lists.xensource.com > Subject: [Xen-users] How does Xen present processors to DomUs? > > Hi, I can''t seem to find this in any documentation, but I''m > curious of how Xen processor reservations work. Do DomUs get > physically allocated several processors to use that remain > just remain a constant number and possibly get shared with > other VMs, or do they really get their own processors that > aren''t touched by anyone else?In short: Yes - both are possible, depending on how you set up your configuration for the domains. You can give Xen free choice to find whichever processor it fancies for the domain, or you can restrict it to say Dom0 always uses CPU#0, Dom1 uses CPU#1,#2 and Dom2 & Dom3 uses CPU#3. Or you could say "Use whatever you like for DomU, as long as it''s not CPU#0", as another example.> > Just to stack two questions, I''m also wondering how safe it > is to mix and match PAE/non-PAE linux kernels under a PAE > xen. PAE should only be necessary in Linux if it''s been > allocated more than 4 gigabytes from Xen, correct? Thanks > much for the replies in advance!At the moment, it''s not possible to use mixes of PAE/no-PAE with para-virtualized Xen. You need to have HWM, hardware virtualization, to allow mixed memory models (2, 3 or 4 level page tables is essentially the choice, although it''s often presented as 32-bit no PAE, 32-bit PAE and 64-bit). The reason for this is that Xen handles the page-table(s) for DomU''s, and they need to match between what the Domain thinks it should be and what Xen actually does - it gets very strange otherwise... ;-) Theoretically, it would be possible to create an interface for running PAE in Xen but not in DomU''s, but at the moment, this is not implemented. Since the interface with HVM (full virtualization) is completely different, this works fine for running "less or equal levels", i.e. 32-bit guests work on all platforms, PAE works if you have at least PAE and 64-bit only works if Xen is 64-bit. [Although I think this is "in theory", because I believe I recently read that running PAE on 32-bit PAE doesn''t ACTUALLY work]. -- Mats> --Greg Cymbalski > > _______________________________________________ > Xen-users mailing list > Xen-users@lists.xensource.com > http://lists.xensource.com/xen-users > >_______________________________________________ Xen-users mailing list Xen-users@lists.xensource.com http://lists.xensource.com/xen-users