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Wed, 02 Sep 2026 08:50:42 -0700 (PDT) Date: Wed, 2 Sep 2026 08:50:41 -0700 In-Reply-To: Mime-Version: 1.0 References: <20260831144802.834315-1-seiden@linux.ibm.com> <20260831144802.834315-4-seiden@linux.ibm.com> <87ik4om4x2.wl-maz@kernel.org> <20260902091351.438647-B-seiden@linux.ibm.com> Message-ID: Subject: Re: [PATCH v7 03/23] KVM: Allow KVM implementations to switch off MMIO independent of Kconfig From: Sean Christopherson To: Steffen Eiden Cc: Marc Zyngier , kvm@vger.kernel.org, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, linux-s390@vger.kernel.org, Alexander Gordeev , Andreas Grapentin , Arnd Bergmann , Catalin Marinas , Christian Borntraeger , Claudio Imbrenda , David Hildenbrand , Friedrich Welter , Fuad Tabba , Gautam Gala , Hariharan Mari , Heiko Carstens , Hendrik Brueckner , Ilya Leoshkevich , Janosch Frank , Joey Gouly , Nico Boehr , Nina Schoetterl-Glausch , Oliver Upton , Paolo Bonzini , Suzuki K Poulose , Sven Schnelle , Ulrich Weigand , Vasily Gorbik , Will Deacon , Zenghui Yu Content-Type: text/plain; charset="us-ascii" X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260902_085043_782170_BFB336F3 X-CRM114-Status: GOOD ( 24.95 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Wed, Sep 02, 2026, Sean Christopherson wrote: > On Wed, Sep 02, 2026, Steffen Eiden wrote: > > On Wed, Sep 02, 2026 at 09:30:17AM +0100, Marc Zyngier wrote: > > > On Wed, 02 Sep 2026 01:38:53 +0100, > > > Sean Christopherson wrote: > > > > > > > > On Mon, Aug 31, 2026, Steffen Eiden wrote: > > > > > Only defining KVM_MMIO is not flexible enough for multi-KVM systems > > > > > where one KVM implements MMIO but others do not. Solve this by > > > > > additionally testing for the non-existence of KVM_NO_MMIO before > > > > > including MMIO code to KVM. > > > > > > > > Ewwwww. IIUC, s390 ends up with two KVM modules, i.e. there really are two different > > > > KVMs being built. Rather than have a KVM_MMIO and NO_KVM_MMIO, I vote very > > > > strongly to drop CONFIG_KVM_MMIO entirely and replace it with __KVM_HAVE_ARCH_MMIO, > > > > a la __KVM_HAVE_ARCH_VM_FREE and others. Tracking two separate macros is going > > > > to be annoying to maintain, and will definitely be confusing for non-s390 people. > > > > > > > > It'll mean linking in an emtpy coalesced_mmio.o on s390 when building without arm64 > > > > support, but I'm guessing that won't be at all common for s390 builds. > > > > > > > > > > I ended-up doing something very similar locally, because I couldn't > > > really make head or tail of the logic while being in holiday mode > > > (which doesn't help). However, I kept the negative logic, so that we > > > don't need to add anything to any architecture but S390 (they are the > > > odd one out here). > > negative makes sense. > > Ya, an opt-out for s390 works for me. Actually, I have a better idea. I'll explain more in the subthread about tracepoints.