Re: [Xen-devel] Linux'es SLUB incompatible with Xen?

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Thu, 2014-09-25 at 11:53 +0100, Jan Beulich wrote:
> >>> On 25.09.14 at 12:08, <Ian.Campbell@xxxxxxxxxx> wrote:
> > On Thu, 2014-09-25 at 09:23 +0100, Jan Beulich wrote:
> >> Linux-Xen maintainers,
> >> 
> >> are you aware that SLUB's use of percpu_cmpxchg16b_double() is
> >> currently incompatible with Xen when run on old, not cmpxchg16b
> >> capable x86-64 CPUs (the problem being this_cpu_cmpxchg16b_emu's
> >> non-paravirtualized use of CLI)? Or is that a simple "we don't care -
> >> don't run Xen on such old systems" (despite everything else, including
> >> the hypervisor, working fine there)?
> > 
> > Out of curiosity what is it about Xen that triggers this, as opposed to
> > failing on native?
> 
> Not sure I understand the question: The emulation code that's being
> used here is fine for native as CLI does what is expected. But for a
> PV Xen guest CLI gets emulated as a privileged NOP, i.e. the needed
> pseudo-atomicity isn't guaranteed.

Sorry, I somehow missed the parenthetical bit in middle of the
paragraph!

Ian.


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