Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after IRQ handling

[Sean Christopherson <seanjc@google.com>]

Apologies for the slow response.

On Wed, Apr 21, 2021, Frederic Weisbecker wrote:
> On Tue, Apr 20, 2021 at 11:26:34PM +0000, Sean Christopherson wrote:
> > On Wed, Apr 21, 2021, Frederic Weisbecker wrote:
> > > On Thu, Apr 15, 2021 at 03:21:00PM -0700, Sean Christopherson wrote:
> > > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > > > index 16fb39503296..e4d475df1d4a 100644
> > > > --- a/arch/x86/kvm/x86.c
> > > > +++ b/arch/x86/kvm/x86.c
> > > > @@ -9230,6 +9230,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> > > >  	local_irq_disable();
> > > >  	kvm_after_interrupt(vcpu);
> > > >  
> > > > +	/*
> > > > +	 * When using tick-based accounting, wait until after servicing IRQs to
> > > > +	 * account guest time so that any ticks that occurred while running the
> > > > +	 * guest are properly accounted to the guest.
> > > > +	 */
> > > > +	if (!vtime_accounting_enabled_this_cpu())
> > > > +		vtime_account_guest_exit();
> > > 
> > > Can we rather have instead:
> > > 
> > > static inline void tick_account_guest_exit(void)
> > > {
> > > 	if (!vtime_accounting_enabled_this_cpu())
> > > 		current->flags &= ~PF_VCPU;
> > > }
> > > 
> > > It duplicates a bit of code but I think this will read less confusing.
> > 
> > Either way works for me.  I used vtime_account_guest_exit() to try to keep as
> > many details as possible inside vtime, e.g. in case the implemenation is tweaked
> > in the future.  But I agree that pretending KVM isn't already deeply intertwined
> > with the details is a lie.
> 
> Ah I see, before 87fa7f3e98a131 the vtime was accounted after interrupts get
> processed. So it used to work until then. I see that ARM64 waits for IRQs to
> be enabled too.
> 
> PPC/book3s_hv, MIPS, s390 do it before IRQs get re-enabled (weird, how does that
> work?)

No idea.  It's entirely possible it doesn't work on one or more of those
architectures.

Based on init/Kconfig, s390 doesn't support tick-based accounting, so I assume
s390 is ok.

  config TICK_CPU_ACCOUNTING
	bool "Simple tick based cputime accounting"
	depends on !S390 && !NO_HZ_FULL

> And PPC/book3s_pr calls guest_exit() so I guess it has interrupts enabled.
> 
> The point is: does it matter to call vtime_account_guest_exit() before or
> after interrupts? If it doesn't matter, we can simply call
> vtime_account_guest_exit() once and for all once IRQs are re-enabled.
> 
> If it does matter because we don't want to account the host IRQs firing at the
> end of vcpu exit, then probably we should standardize that behaviour and have
> guest_exit_vtime() called before interrupts get enabled and guest_exit_tick()
> called after interrupts get enabled. It's probably then beyond the scope of this
> patchset but I would like to poke your opinion on that.
> 
> Thanks.

I don't know.  For x86, I would be ok with simply moving the call to
vtime_account_guest_exit() to after IRQs are enabled.  It would bug me a little
bit that KVM _could_ be more precise when running with
CONFIG_VIRT_CPU_ACCOUNTING_GEN=y, and KVM would still be poking into the details
of vtime_account_guest_exit() to some extent, but overall it would be an
improvement from a code cleanliness perspective.

The problem is I have no clue who, if anyone, deploys KVM on x86 with
CONFIG_VIRT_CPU_ACCOUNTING_GEN=y.  On the other hand, AMD/SVM has always had the
"inaccurate" accounting, and Intel/VMX has been inaccurate since commit
d7a08882a0a4 ("KVM: x86: Unconditionally enable irqs in guest context"), which
amusingly was a fix for an edge case in tick-based accounting.

Anyone have an opinion either way?  I'm very tempted to go with Frederic's
suggestion of moving the time accounting back to where it was, it makes KVM just
a little less ugly.