linux-kernel - Re: [PATCH 3/3] KVM: s390: Use generic VIRT_XFER_TO_GUEST

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Message-ID: <bec71a9ee80b6493da75693808ec846cca56343b.camel@linux.ibm.com>
Date: Wed, 26 Nov 2025 13:01:26 +1100
From: Andrew Donnellan <ajd@...ux.ibm.com>
To: Heiko Carstens <hca@...ux.ibm.com>
Cc: kvm@...r.kernel.org, linux-s390@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Christian Borntraeger
 <borntraeger@...ux.ibm.com>,
        Janosch Frank <frankja@...ux.ibm.com>,
        Claudio Imbrenda <imbrenda@...ux.ibm.com>,
        Vasily Gorbik	
 <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sven
 Schnelle	 <svens@...ux.ibm.com>,
        Nicholas Miehlbradt
 <nicholas@...ux.ibm.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter
 Zijlstra <peterz@...radead.org>,
        Andy Lutomirski	 <luto@...nel.org>,
        David
 Hildenbrand <david@...nel.org>
Subject: Re: [PATCH 3/3] KVM: s390: Use generic VIRT_XFER_TO_GUEST_WORK
 functions

On Tue, 2025-11-25 at 12:16 +0100, Heiko Carstens wrote:
> On Tue, Nov 25, 2025 at 06:45:54PM +1100, Andrew Donnellan wrote:
> > Switch to using the generic infrastructure to check for and handle pending
> > work before transitioning into guest mode.
> > 
> > xfer_to_guest_mode_handle_work() does a few more things than the current
> > code does when deciding whether or not to exit the __vcpu_run() loop. The
> > exittime tests from kvm-unit-tests, in my tests, were +/-3% compared to
> > before this series, which is within noise tolerance.
> 
> ...
> 
> >  		local_irq_disable();
> > +
> > +		xfer_to_guest_mode_prepare();
> > +		if (xfer_to_guest_mode_work_pending()) {
> > +			local_irq_enable();
> > +			rc = kvm_xfer_to_guest_mode_handle_work(vcpu);
> > +			if (rc)
> > +				break;
> > +			local_irq_disable();
> > +		}
> > +
> >  		guest_timing_enter_irqoff();
> >  		__disable_cpu_timer_accounting(vcpu);
> 
> This looks racy: kvm_xfer_to_guest_mode_handle_work() returns with
> interrupts enabled and before interrupts are disabled again more work
> might have been become pending. But that is ignored and guest state is
> entered instead. Why not change the above simply to something like
> this to avoid this:
> 
> again:
> 	local_irq_disable();
> 		xfer_to_guest_mode_prepare();
> 		if (xfer_to_guest_mode_work_pending()) {
> 			local_irq_enable();
> 			rc = kvm_xfer_to_guest_mode_handle_work(vcpu);
> 			if (rc)
> 				break;
> 			goto again;
> 		}
> 
> 		guest_timing_enter_irqoff();
> 		__disable_cpu_timer_accounting(vcpu);
> 
> But maybe I'm missing something?

Agreed, I'll restructure this and respin.

-- 
Andrew Donnellan    OzLabs, ADL Canberra
ajd@...ux.ibm.com   IBM Australia Limited