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

[Sebastian Andrzej Siewior]

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.

>  	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?

> +		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.

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

Sebastian