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Re: [XenPPC] [xenppc-unstable] [ppc] Turn on Save and Restore of VMX reg

On Jun 13, 2006, at 8:43 PM, Segher Boessenkool wrote:

Ok, its a bad comment :).
The point is that, the hypervisor cannot depend on the OSes above us to use VRSAVE at all, so we have to S&R the whole shebang.

Ah, much clearer already.

Do we really have to?  Can't we, for example, define the supervisor
vs. hypervisor ABI wrt. VMX to be "if the hypervisor wants to use
any VMX regs, it better S&R those itself"?  And when switching
supervisor context, just swap-em all out.  Although perhaps this is
just what the full code is doing, souldn't tell from the patch alone :-)

The S&R of FPU and VMX (and SLB while we are at it) occurs only on domain switch. We could come up with optimizations from the domain where the domain can assert that the FPU and VMX registers matter and in that case simply zero them, but that a future optimization.


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