Re: [Xen-devel] [PATCH v9 01/17] VT-d Posted-intterrupt (PI) design

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Tian, Kevin
> Sent: Tuesday, November 24, 2015 2:35 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> Cc: Zhang, Yang Z <yang.z.zhang@xxxxxxxxx>; Jan Beulich
> <jbeulich@xxxxxxxx>; Keir Fraser <keir@xxxxxxx>; Andrew Cooper
> <andrew.cooper3@xxxxxxxxxx>; George Dunlap
> <george.dunlap@xxxxxxxxxxxxx>
> Subject: RE: [PATCH v9 01/17] VT-d Posted-intterrupt (PI) design
> 
> > From: Wu, Feng
> > Sent: Tuesday, November 03, 2015 4:43 PM
> >
> >
> > diff --git a/docs/misc/vtd-pi.txt b/docs/misc/vtd-pi.txt
> > new file mode 100644
> > index 0000000..f9b4637
> > --- /dev/null
> > +++ b/docs/misc/vtd-pi.txt
> > @@ -0,0 +1,329 @@
> > +Authors: Feng Wu <feng.wu@xxxxxxxxx>
> > +
> > +VT-d Posted-interrupt (PI) design for XEN
> > +
> > +Background
> > +==========
> > +With the development of virtualization, there are more and more device
> > +assignment requirements. However, today when a VM is running with
> > +assigned devices (such as, NIC), external interrupt handling for the
> assigned
> > +devices always needs VMM intervention.
> > +
> > +VT-d Posted-interrupt is a more enhanced method to handle interrupts
> > +in the virtualization environment. Interrupt posting is the process by
> > +which an interrupt request is recorded in a memory-resident
> > +posted-interrupt-descriptor structure by the root-complex, followed by
> > +an optional notification event issued to the CPU complex.
> 
> Some clarification required here. Is "Interrupt Posting" only used to
> represent the process of VT-d Posted-interrupt, or also for CPU-side
> posting through IPI? From above context, and later explanation about
> "Processor Support" and "Root-complex Support", looks the former
> is true. Then how do we call CPU-side posting?

Thanks for the comments. "Interrupt posting" in fact contains vt-d side
and cpu side. So I will reword this to make it clear.

Thanks,
Feng

> 
> I think a one-sentence definition about those terms in the start would
> be helpful.
> 
> > +
> > +With VT-d Posted-interrupt we can get the following advantages:
> > +- Direct delivery of external interrupts to running vCPUs without VMM
> > +intervention
> > +- Decrease the interrupt migration complexity. On vCPU migration,
> software
> > +can atomically co-migrate all interrupts targeting the migrating vCPU. For
> > +virtual machines with assigned devices, migrating a vCPU across pCPUs
> > +either incurs the overhead of forwarding interrupts in software (e.g. via
> VMM
> > +generated IPIs), or complexity to independently migrate each interrupt
> targeting
> > +the vCPU to the new pCPU. However, after enabling VT-d PI, the
> destination vCPU
> > +of an external interrupt from assigned devices is stored in the IRTE (i.e.
> > +Posted-interrupt Descriptor Address), when vCPU is migrated to another
> pCPU,
> > +we will set this new pCPU in the 'NDST' filed of Posted-interrupt
> descriptor, this
> > +make the interrupt migration automatic.
> > +
> > +Here is what Xen currently does for external interrupts from assigned
> devices:
> > +
> > +When a VM is running and an external interrupt from an assigned device
> occurs
> > +for it. VM-EXIT happens, then:
> > +
> > +vmx_do_extint() --> do_IRQ() --> __do_IRQ_guest() --> hvm_do_IRQ_dpci()
> -->
> > +raise_softirq_for(pirq_dpci) --> raise_softirq(HVM_DPCI_SOFTIRQ)
> > +
> > +softirq HVM_DPCI_SOFTIRQ is bound to dpci_softirq()
> > +
> > +dpci_softirq() --> hvm_dirq_assist() --> vmsi_deliver_pirq() -->
> vmsi_deliver() -->
> > +vmsi_inj_irq() --> vlapic_set_irq()
> > +
> > +vlapic_set_irq() does the following things:
> > +1. If CPU-side posted-interrupt is supported, call
> vmx_deliver_posted_intr() to deliver
> > +the virtual interrupt via posted-interrupt infrastructure.
> > +2. Else if CPU-side posted-interrupt is not supported, set the related vIRR
> in vLAPIC
> > +page and call vcpu_kick() to kick the related vCPU. Before VM-Entry,
> vmx_intr_assist()
> > +will help to inject the interrupt to guests.
> > +
> > +However, after VT-d PI is supported, when a guest is running in non-root
> and an
> > +external interrupt from an assigned device occurs for it. No VM-Exit is
> needed,
> 
> ". No VM-Exit ..." -> ", no VM-Exit..."
> 
> > +the guest can handle this totally in non-root mode, thus avoiding all the
> above
> > +code flow.
> > +
> > +Posted-interrupt Introduction
> > +========================
> > +There are two components to the Posted-interrupt architecture:
> 
> "to the" -> "in the"
> 
> 
> Thanks
> Kevin

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