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

[Shrikanth Hegde]

On 4/24/25 20:12, Sebastian Andrzej Siewior wrote:

Thanks Sebastian for taking a look.

> On 2025-04-21 15:58:36 [+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
> > @@ -80,8 +80,8 @@
> >   #include <asm/ultravisor.h>
> >   #include <asm/dtl.h>
> >   #include <asm/plpar_wrappers.h>
> > -
> >   #include <trace/events/ipi.h>
> > +#include <linux/entry-kvm.h>
> >   
> >   #include "book3s.h"
> >   #include "book3s_hv.h"
> > @@ -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.

> >   	kvmppc_update_vpas(vcpu);
> >   
> > @@ -5097,10 +5097,11 @@ static int kvmppc_vcpu_run_hv(struct kvm_vcpu *vcpu)
> >   		return -EINVAL;
> >   	}
> >   
> > -	/* No need to go into the guest when all we'll do is come back out */
> > -	if (signal_pending(current)) {
> > -		run->exit_reason = KVM_EXIT_INTR;
> > -		return -EINTR;
> > +	/* use generic frameworks to handle signals, need_resched  */
> > +	if (__xfer_to_guest_mode_work_pending()) {
> > +		r = xfer_to_guest_mode_handle_work(vcpu);
> This could be unconditional.
>
> > +		if (r)
> > +			return r;
> >   	}
> >   
> >   #ifdef CONFIG_PPC_TRANSACTIONAL_MEM
> > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
> > index 153587741..4ff334532 100644
> > --- 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.

Will update in v2.

> > +		if (r)
> > +			return r;
> > +	}
> > +
> >   	WARN_ON(irqs_disabled());
> >   	hard_irq_disable();
> >   
> >   	while (true) {
> > -		if (need_resched()) {
> > -			local_irq_enable();
> > -			cond_resched();
> > -			hard_irq_disable();
> > -			continue;
> > -		}
> > -
> > -		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;

> > -		}
> > -
> >   		vcpu->mode = IN_GUEST_MODE;
> >   
> >   		/*
>
> Sebastian