linux-kernel - Re: [PATCH 1/4] KVM: delete .change

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Message-ID: <ZiJ4bqrBUPM0E8iq@google.com>
Date: Fri, 19 Apr 2024 06:58:06 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Will Deacon <will@...nel.org>
Cc: Marc Zyngier <maz@...nel.org>, Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org, 
	kvm@...r.kernel.org, Oliver Upton <oliver.upton@...ux.dev>, 
	Tianrui Zhao <zhaotianrui@...ngson.cn>, Bibo Mao <maobibo@...ngson.cn>, 
	Thomas Bogendoerfer <tsbogend@...ha.franken.de>, Nicholas Piggin <npiggin@...il.com>, 
	Anup Patel <anup@...infault.org>, Atish Patra <atishp@...shpatra.org>, 
	Andrew Morton <akpm@...ux-foundation.org>, David Hildenbrand <david@...hat.com>, 
	linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev, 
	loongarch@...ts.linux.dev, linux-mips@...r.kernel.org, 
	linuxppc-dev@...ts.ozlabs.org, kvm-riscv@...ts.infradead.org, 
	linux-mm@...ck.org, linux-trace-kernel@...r.kernel.org, 
	linux-perf-users@...r.kernel.org
Subject: Re: [PATCH 1/4] KVM: delete .change_pte MMU notifier callback

On Fri, Apr 19, 2024, Will Deacon wrote:
> > @@ -663,10 +669,22 @@ static __always_inline kvm_mn_ret_t __kvm_handle_hva_range(struct kvm *kvm,
> >  					break;
> >  			}
> >  			r.ret |= range->handler(kvm, &gfn_range);
> > +
> > +		       /*
> > +			* Use a precise gfn-based TLB flush when possible, as
> > +			* most mmu_notifier events affect a small-ish range.
> > +			* Fall back to a full TLB flush if the gfn-based flush
> > +			* fails, and don't bother trying the gfn-based flush
> > +			* if a full flush is already pending.
> > +			*/
> > +		       if (range->flush_on_ret && !need_flush && r.ret &&
> > +			   kvm_arch_flush_remote_tlbs_range(kvm, gfn_range.start,
> > +							    gfn_range.end - gfn_range.start + 1))
> 
> What's that '+ 1' needed for here?

 (a) To see if you're paying attention.
 (b) Because more is always better.
 (c) Because math is hard.
 (d) Because I haven't tested this.
 (e) Both (c) and (d).