Re: [PATCH] x86/apicv: enhance posted-interrupt processing

["Tian, Kevin" <kevin.tian@intel.com>]

> From: Xuquan (Quan Xu) [mailto:xuquan8@huawei.com]
> Sent: Tuesday, February 21, 2017 10:49 AM
> 
> On February 20, 2017 4:24 PM, Chao Gao wrote:
> >On Mon, Feb 20, 2017 at 11:25:29AM +0000, Xuquan (Quan Xu) wrote:
> >>On February 18, 2017 12:33 AM, Jan Beulich wrote:
> >>>>>> On 17.02.17 at 09:49, <chao.gao@intel.com> wrote:
> >>>> On Fri, Feb 17, 2017 at 09:37:45AM +0000, Xuquan (Quan Xu) wrote:
> >>>>>From a589074281cc22a30ed75a5bccba60e83d2312a6 Mon Sep 17
> >>>00:00:00 2001
> >>>>>From: Quan Xu <xuquan8@huawei.com>
> >>>>>Date: Sat, 18 Feb 2017 09:27:37 +0800
> >>>>>Subject: [PATCH] x86/apicv: enhance posted-interrupt processing
> >>>>>
> >>>>>If guest is already in non-root mode, an posted interrupt will be
> >>>>>directly delivered to guest (leaving softirq being set w/o actually
> >>>>>incurring a VM-Exit - breaking desired softirq behavior).
> >>>>>Then further posted interrupts will skip the IPI, stay in PIR and
> >>>>>not noted until another VM-Exit happens.
> >>>>>
> >>>>>Remove the softirq set. Actually since it's an optimization for less
> >>>>>IPIs, check softirq_pending(cpu) directly instead of sticking to one
> >>>>>bit only.
> >>>>>
> >>>>>Signed-off-by: Quan Xu <xuquan8@huawei.com>
> >>>>>---
> >>>>> xen/arch/x86/hvm/vmx/vmx.c | 3 +--
> >>>>> 1 file changed, 1 insertion(+), 2 deletions(-)
> >>>>>
> >>>>>diff --git a/xen/arch/x86/hvm/vmx/vmx.c
> >b/xen/arch/x86/hvm/vmx/vmx.c
> >>>>>index 61925cf..3887c32 100644
> >>>>>--- a/xen/arch/x86/hvm/vmx/vmx.c
> >>>>>+++ b/xen/arch/x86/hvm/vmx/vmx.c
> >>>>>@@ -1846,8 +1846,7 @@ static void
> >>>__vmx_deliver_posted_interrupt(struct vcpu *v)
> >>>>>     {
> >>>>>         unsigned int cpu = v->processor;
> >>>>>
> >>>>>-        if ( !test_and_set_bit(VCPU_KICK_SOFTIRQ,
> >>>&softirq_pending(cpu))
> >>>>>-             && (cpu != smp_processor_id()) )
> >>>>>+        if ( !softirq_pending(cpu) && (cpu != smp_processor_id()) )
> >>>> HI, Quan.
> >>>> Is there a situation that we need set VCPU_KICK_SOFTIRQ. For
> >>>> example, after vmx_intr_assist(), a interrupt happened and its
> >>>> handler called this function to deliver interrupt to current vcpu.
> >>>> In that case, the interrupt would not be injected to guest before
> >>>> this VM-entry for we don't generate a softirq and don't send a self-IPI
> >to current vcpu.
> >>>
> >>
> >>Chao, __iiuc__, your question may be from the comments of
> >xen/arch/x86/hvm/vmx/vmx.c :: pi_notification_interrupt() ..
> >>IF VT-d PI is enabled,
> >>   VCPU_KICK_SOFTIRQ bit is set by ' raise_softirq(VCPU_KICK_SOFTIRQ)',
> >at the end of pi_notification_interrupt()..
> >>Else
> >>  Is it possible for your case?
> >>
> >If vcpu is in root mode and is to do VM-entry, it has synced PIR to vIRR.
> >Now a interrupt (e.g. PMU_APIC_VECTOR) happens. Thus it goes following
> >the path pmu_apic_interrupt->vpmu_do_interrupt->vlapic_set_irq(assume
> >it will inject a interrupt to current vcpu)
> >-> vmx_deliver_posted_intr( set one bit in PIR )->
> >-> __vmx_deliver_posted_interrupt
> >Assuming that there is no softirq pending, the code after change doesn't
> >generate a IPI for (cpu == smp_processor_id()). In this case, this interrupt
> >would not be injected to guest before this VM-entry.
> >Although there are many assumption in the explaination, I think it may be
> >possible.
> >
> 
> So far, I agree to add VCPU_KICK_SOFTIRQ bit in a nice way..
> Even we have checked whether the vCPU is running or not at the beginning of
> __vmx_deliver_posted_interrupt(),
> we can't grantee the vcpu is still in guest mode at the point to call this IPI..
> as in an extreme case, at the point to call this IPI, all of vCPUs are in root-mode, the
> posted-interrupt
> won't be delivered..

I don't understand of your concern of whether guest is in guest mode here.
The purpose of this function is not to guarantee posted-interrupt is always
used (cannot unless you pause remote cpu). It's just a best effort. If target 
vcpu is in root mode, then this IPI causes a real interrupt on remote cpu as 
notification (then the handler pi_notification_interrupt you copied earlier will 
jump in to help).


Thanks
Kevin

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