Re: [Xen-devel] [PATCH v15 01/11] multicall: add no preemption ability between two calls

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.09.14 at 11:43, <andrew.cooper3@xxxxxxxxxx> wrote:
> Actually, on further thought, using multicalls like this cannot possibly
> be correct from a functional point of view.
> 
> Even with the no preempt flag between a wrmsr/rdmsr hypercall pair,
> there is no guarantee that accesses to remote cpus msrs won't interleave
> with a different natural access, clobbering the results of the wrmsr.
> 
> However this is solved, the wrmsr/rdmsr pair *must* be part of the same
> synchronous thread of execution on the appropriate cpu.  You can trust
> that interrupts won't play with these msrs, but you absolutely can't
> guarantee that IPI/wrmsr/IPI/rdmsr will work.

Not sure I follow, particularly in the context of the white listing of
MSRs permitted here (which ought to not include anything the
hypervisor needs control over).

Jan


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