Re: [Xen-devel] [PATCH] xen/arm: Handle platforms with edge-triggered virtual timer

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-12-02 at 14:08 +0000, Julien Grall wrote:
> Hi Ian,
> 
> On 02/12/14 13:54, Ian Campbell wrote:
> > On Fri, 2014-11-28 at 15:17 +0000, Julien Grall wrote:
> >> Some platforms (such as Xgene and ARMv8 models) use an edge-triggered 
> >> interrupt
> >> for the virtual timer. Even if the timer output signal is masked in the
> >> context switch, the GIC will keep track that of any interrupts raised
> >> while IRQs are disabled. As soon as IRQs are re-enabled, the virtual
> >> interrupt timer will be injected to Xen.
> >>
> >> If an idle vVCPU was scheduled next then the interrupt handler doesn't
> >> expect to the receive the IRQ and will crash:
> >>
> >> (XEN)    [<0000000000228388>] _spin_lock_irqsave+0x28/0x94 (PC)
> >> (XEN)    [<0000000000228380>] _spin_lock_irqsave+0x20/0x94 (LR)
> >> (XEN)    [<0000000000250510>] vgic_vcpu_inject_irq+0x40/0x1b0
> >> (XEN)    [<000000000024bcd0>] vtimer_interrupt+0x4c/0x54
> >> (XEN)    [<0000000000247010>] do_IRQ+0x1a4/0x220
> >> (XEN)    [<0000000000244864>] gic_interrupt+0x50/0xec
> >> (XEN)    [<000000000024fbac>] do_trap_irq+0x20/0x2c
> >> (XEN)    [<0000000000255240>] hyp_irq+0x5c/0x60
> >> (XEN)    [<0000000000241084>] context_switch+0xb8/0xc4
> >> (XEN)    [<000000000022482c>] schedule+0x684/0x6d0
> >> (XEN)    [<000000000022785c>] __do_softirq+0xcc/0xe8
> >> (XEN)    [<00000000002278d4>] do_softirq+0x14/0x1c
> >> (XEN)    [<0000000000240fac>] idle_loop+0x134/0x154
> >> (XEN)    [<000000000024c160>] start_secondary+0x14c/0x15c
> >> (XEN)    [<0000000000000001>] 0000000000000001
> >>
> >> The proper solution is to context switch the virtual interrupt state at
> >> the GIC level. This would also avoid masking the output signal which
> >> requires specific handling in the guest OS and more complex code in Xen
> >> to deal with EOIs, and so is desirable for that reason too.
> >>
> >> Sadly, this solution requires some refactoring which would not be
> >> suitable for a freeze exception for the Xen 4.5 release.
> >>
> >> For now implement a temporary solution which ignores the virtual timer
> >> interrupt when the idle VCPU is running.
> >>
> > 
> > When we reschedule the vcpu which caused the spurious interrupt, the IRQ
> > will definitely trigger again for real, right?
> 
> Xen arms a timer when the domain is saved. As we received an unexpected
> interrupt that means the timer expires which will make Xen injected the
> virtual timer interrupt (see virt_timer_expired).

Are we sure there is no race here where the software timer doesn't fire
because it appears to be in the past or something?

That would correspond to the sequence:
   disable interrupts
   h/w timer expires, interrupt raised but masked
   calculate timeout for s/w timeout => -ve.

Perhaps Xen s/w timers in the past fire immediately?



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