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Thu, 03 Sep 2026 07:30:36 -0700 (PDT) Date: Thu, 3 Sep 2026 07:30:35 -0700 In-Reply-To: <20260903114241.33034-C-seiden@linux.ibm.com> Mime-Version: 1.0 References: <20260831144802.834315-1-seiden@linux.ibm.com> <20260831144802.834315-3-seiden@linux.ibm.com> <20260902075028.231001-D-seiden@linux.ibm.com> <20260903114241.33034-C-seiden@linux.ibm.com> Message-ID: Subject: Re: [PATCH v7 02/23] KVM: Make device name configurable From: Sean Christopherson To: Steffen Eiden Cc: 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 , Marc Zyngier , 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-20260903_073038_697402_DF04BE2B X-CRM114-Status: GOOD ( 25.26 ) 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 Thu, Sep 03, 2026, Steffen Eiden wrote: > On Wed, Sep 02, 2026 at 09:14:21AM -0700, Sean Christopherson wrote: > > And taking things a few steps further, we can solve the MMIO issue in a more > > elegant way, and eliminate the runtime string building in this patch (after looking > > more closely, that code needs to be jettisoned no matter what, there's simply no > > reason to specify the names at runtime since they're separate compilation units). > > > > Rather than splatter #defines throughout header files, deal with the bulk of the > > pain in Makefile.kvm. By feeding conditionals into Makefile.kvm, the s390+arm64 > > build can easily omit coalesced_mmio.o and async_pf.o, define __KVM_HAVE_ARCH_MMIO > > programatically without having to change other architectures, and solve the naming > > stuff. > > Yes, this is a great idea. Thank you. I second you, this looks more clean > and stable than the stuff we came up with :) Looking more at arch/s390/kvm/Kconfig and virt/kvm/Kconfig, we might need/want to to build out infrastructure to handle this sort of thing in a more generic fashion? Which probably isn't that much infrastructure? It's more just changing how arch code communicates with common KVM? I.e. instead of providing boolean configs in virt/kvm/Kconfig, formalize communicating HAVE-type macros through the Makefile. That might even be a net positive in the long run, as it will make it easier to provide defaults for the common cases. I say that because unless there's magic I'm unaware of these Kconfigs also needs to be configured per-KVM, not per-kernel: - KVM_MMU_LOCKLESS_AGING, otherwise aging on arm64 will unintentionally be done outside of mmu_lock. - KVM_GENERIC_PRE_FAULT_MEMORY, so that arm64 doesn't need to provide a stub for something it doesn't support - HAVE_KVM_MSI, because presumably it's needed for arm64 support. - HAVE_KVM_READONLY_MEM, same story as HAVE_KVM_MSI. Which isn't _that_ many knobs, but add in KVM_MMIO, KVM_ASYNC_PF, and KVM_ASYNC_PF_SYNC, and it's enough that I think we should think about the big picture and not play whack-a-mole. Actually, thinking about this more, what I've proposed here, plus the pattern of #define-ing macros in arch-specific kvm_host.h files, should suffice. For things like __KVM_HAVE_ARCH_VM_FREE, it absolutely makes sense to #define the macro in kvm_host.h since it's very directly tied to an arch callback. Whereas with KVM_MIO and KVM_ASYNC_PF, because they enable compilation of C files, it makes sense to define them in Makefiles. So I think the "gap" is purely that the four knobs listed above need handling (and arguably KVM_MMU_LOCKLESS_AGING is ok as-proposed). But I do think we should try our best to be more thoughtful than we usually are when deciding how to enable common code, because obviously s390+arm64 is adding wrinkles no one has had to deal with before.