linux-kernel - Re: [PATCH 1/2] powerpc: kvm: use generic transfer to guest mode work

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Message-ID: <20250424183811.7_MLThpt@linutronix.de>
Date: Thu, 24 Apr 2025 20:38:11 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Shrikanth Hegde <sshegde@...ux.ibm.com>
Cc: maddy@...ux.ibm.com, linuxppc-dev@...ts.ozlabs.org, npiggin@...il.com,
	christophe.leroy@...roup.eu, gautam@...ux.ibm.com,
	vaibhav@...ux.ibm.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] powerpc: kvm: use generic transfer to guest mode work

On 2025-04-24 21:27:59 [+0530], Shrikanth Hegde wrote:
> > > diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> > > index 19f4d298d..123539642 100644
> > > --- a/arch/powerpc/kvm/book3s_hv.c
> > > +++ b/arch/powerpc/kvm/book3s_hv.c
> > > @@ -4901,7 +4901,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
> > >   	}
> > >   	if (need_resched())
> > > -		cond_resched();
> > > +		schedule();
> > 
> 
> 
> > This looks unrelated and odd. I don't why but this should be a
> > cond_resched() so it can be optimized away on PREEMPT kernels.
> 
> This is needed, otherwise KVM on powerVM setup gets stuck on preempt=full/lazy.

But this makes no sense. On preempt=full the cond_resched() gets patched
out while schedule() doesn't. Okay, this explains the stuck.

On preempt=full need_resched() should not return true because the
preemption happens right away. Unless you are in a preempt-disabled
or interrupt disabled section. But any of the conditions can't be true
because in both cases you can't invoke schedule(). So you must have had
a wake up on the local CPU which sets need-resched but the schedule()
was delayed for some reason. Once that need-resched bit is observed by
a remote CPU then it won't send an interrupt for a scheduling request
because it should happen any time soon… This should be fixed.

If you replace the above with preempt_disable(); preempt_enable() then it
should also work…

…
> > > --- a/arch/powerpc/kvm/powerpc.c
> > > +++ b/arch/powerpc/kvm/powerpc.c
> > > @@ -34,6 +34,7 @@
> > >   #endif
> > >   #include <asm/ultravisor.h>
> > >   #include <asm/setup.h>
> > > +#include <linux/entry-kvm.h>
> > >   #include "timing.h"
> > >   #include "../mm/mmu_decl.h"
> > > @@ -80,24 +81,17 @@ int kvmppc_prepare_to_enter(struct kvm_vcpu *vcpu)
> > >   {
> > >   	int r;
> > > +	/* use generic framework to handle need resched and signals */
> > > +	if (__xfer_to_guest_mode_work_pending()) {
> > > +		r = xfer_to_guest_mode_handle_work(vcpu);
> > 
> > there is nothing special you do checking and handling the work. Couldn't
> > you invoke xfer_to_guest_mode_handle_work() unconditionally?
> > 
> 
> I followed what was in arch/x86/kvm/x86.c. Since xfer_to_guest_mode_handle_work does the same check
> it makes sense to call it without checks too.

Yeah but I guess x86 did some other updates, too.

> Will update in v2.
> 
…
> > > -
> > > -		if (signal_pending(current)) {
> > > -			kvmppc_account_exit(vcpu, SIGNAL_EXITS);
> > > -			vcpu->run->exit_reason = KVM_EXIT_INTR;
> > > -			r = -EINTR;
> > > -			break;
> > 
> > I don't how this works but couldn't SIGNAL_EXITS vanish now that it
> > isn't updated anymore? The stat itself moves in kvm_handle_signal_exit()
> > to a different counter so it is not lost. The reader just needs to look
> > somewhere else for it.
> 
> ok. thanks for pointing out.
> 
> AFAIU it is updating the stats mostly. But below could keep the stats happy.
> I will update that in v2.
> 
>         if (__xfer_to_guest_mode_work_pending()) {
>                 r = xfer_to_guest_mode_handle_work(vcpu);
> +               /* generic framework doesn't update ppc specific stats*/
> +               if (r == -EINTR)
> +                       kvmppc_account_exit(vcpu, SIGNAL_EXITS);
>                 if (r)
>                         return r;

Either that or you rip it out entirely but that is not my call.

Sebastian