Re: [Xen-devel] [RFC v1 12/15] vmx: Properly handle notification event when vCPU is running

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Apr 03, 2015 at 02:00:24AM +0000, Wu, Feng wrote:
> 
> 
> > -----Original Message-----
> > From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> > Sent: Friday, April 03, 2015 3:15 AM
> > To: Tian, Kevin
> > Cc: Wu, Feng; Zhang, Yang Z; xen-devel@xxxxxxxxxxxxx; keir@xxxxxxx;
> > JBeulich@xxxxxxxx
> > Subject: Re: [Xen-devel] [RFC v1 12/15] vmx: Properly handle notification 
> > event
> > when vCPU is running
> > 
> > On Thu, Apr 02, 2015 at 06:08:12AM +0000, Tian, Kevin wrote:
> > > > From: Wu, Feng
> > > > Sent: Friday, March 27, 2015 12:58 PM
> > > >
> > > >
> > > >
> > > > > -----Original Message-----
> > > > > From: Zhang, Yang Z
> > > > > Sent: Friday, March 27, 2015 12:44 PM
> > > > > To: Wu, Feng; xen-devel@xxxxxxxxxxxxx
> > > > > Cc: JBeulich@xxxxxxxx; keir@xxxxxxx; Tian, Kevin
> > > > > Subject: RE: [RFC v1 12/15] vmx: Properly handle notification event 
> > > > > when
> > > > vCPU
> > > > > is running
> > > > >
> > > > > Wu, Feng wrote on 2015-03-27:
> > > > > >
> > > > > >
> > > > > > Zhang, Yang Z wrote on 2015-03-25:
> > > > > >> when vCPU is running
> > > > > >>
> > > > > >> Wu, Feng wrote on 2015-03-25:
> > > > > >>> When a vCPU is running in Root mode and a notification event has
> > > > > >>> been injected to it. we need to set VCPU_KICK_SOFTIRQ for the
> > > > > >>> current cpu, so the pending interrupt in PIRR will be synced to
> > > > > >>> vIRR before
> > 
> > This would imply that we had VMEXIT-ed due to pending interrupt? And we
> > end up calling 'do_IRQ'? If so then the DPCI_SOFTIRQ ends up being set
> > and you stll end up calling the softirq code?
> 
> No. 
> 
> Here is the scenario for the description of this patch:
> 
> When vCPU is running in root-mode (such as via hypercall, or any other
> reasons which can result in VM-Exit), and before vCPU is back to non-root,
> external interrupts happen. Notice that the VM-exit is not caused by this
> external interrupt.

Thank you for the explanation. You might want to add that in the commit
along with the explanation of the code flow below!

> 
> Thanks,
> Feng
> 
> > 
> > > > > > VM-Exit in time.
> > > > > >>
> > > > > >> Shouldn't the pending interrupt be synced unconditionally before 
> > > > > >> next
> > > > > >> vmentry? What happens if we didn't set the softirq?
> > > > > >
> > > > > > If we didn't set the softirq in the notification handler, the
> > > > > > interrupts happened exactly before VM-entry cannot be delivered to
> > > > > > guest at this time. Please see the following code fragments from
> > > > > > xen/arch/x86/hvm/vmx/entry.S: (pls pay attention to the comments)
> > > > > >
> > > > > > .Lvmx_do_vmentry
> > > > > >
> > > > > > ......
> > > > > >             /* If Vt-d engine issues a notification event here,
> > > > > >          * it cannot be delivered to guest during this VM-entry
> > > > > >          * without raising the softirq in notification handler. */
> > > > > >         cmp  %ecx,(%rdx,%rax,1)
> > > > > >         jnz  .Lvmx_process_softirqs
> > > > > > ......
> > > > > >
> > > > > >         je   .Lvmx_launch
> > > > > > ......
> > > > > >
> > > > > >
> > > > > > .Lvmx_process_softirqs:
> > > > > >         sti
> > > > > >         call do_softirq
> > > > > >         jmp  .Lvmx_do_vmentry
> > > > >
> > > > > You are right! This helps me to recall why raise the softirq when 
> > > > > delivering
> > > > the
> > > > > PI.
> > > >
> > > > Yes, __vmx_deliver_posted_interrupt() is the software way to deliver 
> > > > PI, it
> > sets
> > > > the
> > > > softirq for this purpose, however, when VT-d HW delivers PI, we have no
> > > > control to
> > > > the HW itself, hence we need to set this softirq in the Notification 
> > > > Event
> > > > handler.
> > > >
> > >
> > > could you include this information in the comment so others can easily
> > > understand this requirement? from code you only mentioned VCPU_KICK
> > > _SOFTIRQ is required, but how it leads to PIRR->VIRR sync is not 
> > > explained.
> > >
> > > Thanks
> > > Kevin
> > >
> > > _______________________________________________
> > > Xen-devel mailing list
> > > Xen-devel@xxxxxxxxxxxxx
> > > http://lists.xen.org/xen-devel

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