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

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Message-ID: <365ba747-9311-45ab-b798-80ba4e578796@linux.ibm.com>
Date: Thu, 24 Apr 2025 21:27:59 +0530
From: Shrikanth Hegde <sshegde@...ux.ibm.com>
To: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
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 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