Re: [PATCH] KVM: x86: Avoid busy loops over uninjectable pending APIC timers

[Gleb Natapov <gleb@redhat.com>]

On Wed, Mar 20, 2013 at 08:19:13PM -0300, Marcelo Tosatti wrote:
> On Wed, Mar 20, 2013 at 11:32:38PM +0200, Gleb Natapov wrote:
> > On Wed, Mar 20, 2013 at 05:03:19PM -0300, Marcelo Tosatti wrote:
> > > On Wed, Mar 20, 2013 at 04:30:33PM -0300, Marcelo Tosatti wrote:
> > > > On Sun, Mar 17, 2013 at 12:47:17PM +0200, Gleb Natapov wrote:
> > > > > On Sun, Mar 17, 2013 at 11:45:34AM +0100, Jan Kiszka wrote:
> > > > > > On 2013-03-17 09:47, Gleb Natapov wrote:
> > > > > > > On Sat, Mar 16, 2013 at 09:49:07PM +0100, Jan Kiszka wrote:
> > > > > > >> From: Jan Kiszka <jan.kiszka@siemens.com>
> > > > > > >>
> > > > > > >> If the guest didn't take the last APIC timer interrupt yet and generates
> > > > > > >> another one on top, e.g. via periodic mode, we do not block the VCPU
> > > > > > >> even if the guest state is halted. The reason is that
> > > > > > >> apic_has_pending_timer continues to return a non-zero value.
> > > > > > >>
> > > > > > >> Fix this busy loop by taking the IRR content for the LVT vector in
> > > > > > >> apic_has_pending_timer into account.
> > > > > > >>
> > > > > > > Just drop coalescing tacking for lapic interrupt. After posted interrupt
> > > > > > > will be merged __apic_accept_irq() will not longer return coalescing
> > > > > > > information, so the code will be dead anyway.
> > > > > > 
> > > > > > That requires the RTC decoalescing series to go first to avoid a
> > > > > > regression, no? Then let's postpone this topic for now.
> > > > > > 
> > > > > Yes, but decoalescing will work only for RTC :(
> > > > 
> > > > Are you proposing to drop LAPIC interrupt reinjection? 
> > > 
> > > Since timer handling and injection is VCPU-local for LAPIC,
> > > __apic_accept_irq can (and must) return coalesced information (cannot
> > > drop LAPIC interrupt reinjection).
> > > 
> > Why can't we drop LAPIC interrupt reinjection? Proposed posted interrupt
> > patches do not properly check for interrupt coalescing even for
> > VCPU-local injection.
> > 
> > --
> > 			Gleb.
> 
> Because older Linux guests depend on reinjection for proper timekeeping.
Which versions? Those without kvmclock? Can we make them use PIT instead?
Posted interrupts going to break them.

--
			Gleb.