[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PROPOSAL] Event channel for SMP-VMs: per-vCPU or per-OS?
>>> On 29.10.13 at 10:49, Luwei Cheng <chengluwei@xxxxxxxxx> wrote: > On Tue, Oct 29, 2013 at 5:34 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote: >> >>> On 29.10.13 at 10:02, Luwei Cheng <chengluwei@xxxxxxxxx> wrote: >> > On Tue, Oct 29, 2013 at 4:19 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote: >> >> >>> On 29.10.13 at 03:56, Luwei Cheng <chengluwei@xxxxxxxxx> wrote: >> >> > Hmm.. though all vCPUs can serve the events, the hypervisor delivers the >> >> > event to only "one" vCPU at at time, so only that vCPU can see this >> >> > event. >> >> > Analytically no race condition will be introduced. >> >> >> >> No - an event is globally pending (at least in the old model, the >> >> situation is better with the new FIFO model), i.e. if more than >> >> one of the guest's vCPU-s allowed to service it would be looking >> >> at it simultaneously, they'd still need to arbitrate which one >> >> ought to handle it. >> >> >> >> So your proposed extension might need to be limited to the >> >> FIFO model. >> > >> > Thanks for your reply. Yes, you are right. My prior description was >> > incorrect. >> > When there are more than one vCPUs picking the event, even without >> > arbitrary, will it cause "correctness" problem? After the event is >> served by >> > the first entered vCPU, and the rest vCPUs just have noting to do in the >> > event handler (no much harm). >> >> That really depends on the handler. Plus it might be a performance >> and/or latency issue to run handlers that don't need to be run. > > I think the situation is much like IO-APIC routing in physical SMP > systems: Indeed, yet you draw the wrong conclusion. > in logical destination mode, all processors can serve I/O interrupts. But only one gets delivered any individual instance - there is arbitration being done in hardware. Jan _______________________________________________ Xen-devel mailing list Xen-devel@xxxxxxxxxxxxx http://lists.xen.org/xen-devel
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