Re: [PATCH v7 03/23] KVM: Allow KVM implementations to switch off MMIO independent of Kconfig

[Marc Zyngier <maz@kernel.org>]

On Wed, 02 Sep 2026 01:38:53 +0100,
Sean Christopherson <seanjc@google.com> 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).

Untested hack follows.

	M.

diff --git a/arch/arm64/kvm/Kconfig b/arch/arm64/kvm/Kconfig
index 449154f9a4852..dd138b730217e 100644
--- a/arch/arm64/kvm/Kconfig
+++ b/arch/arm64/kvm/Kconfig
@@ -22,7 +22,6 @@ menuconfig KVM
 	select KVM_COMMON
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select HAVE_KVM_CPU_RELAX_INTERCEPT
-	select KVM_MMIO
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
 	select VIRT_XFER_TO_GUEST_WORK
 	select KVM_VFIO
diff --git a/arch/loongarch/kvm/Kconfig b/arch/loongarch/kvm/Kconfig
index 15da2d88c0c10..f03872ba0ba8c 100644
--- a/arch/loongarch/kvm/Kconfig
+++ b/arch/loongarch/kvm/Kconfig
@@ -28,7 +28,6 @@ config KVM
 	select KVM_COMMON
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
 	select KVM_GENERIC_HARDWARE_ENABLING
-	select KVM_MMIO
 	select VIRT_XFER_TO_GUEST_WORK
 	select SCHED_INFO
 	select GUEST_PERF_EVENTS if PERF_EVENTS
diff --git a/arch/mips/kvm/Kconfig b/arch/mips/kvm/Kconfig
index b1b9a1d677581..c65596fdf54f4 100644
--- a/arch/mips/kvm/Kconfig
+++ b/arch/mips/kvm/Kconfig
@@ -22,7 +22,6 @@ config KVM
 	select EXPORT_UASM
 	select KVM_COMMON
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
-	select KVM_MMIO
 	select KVM_GENERIC_HARDWARE_ENABLING
 	select HAVE_KVM_READONLY_MEM
 	help
diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
index b6bc2fc86dca6..c5565d6fb27e4 100644
--- a/arch/powerpc/kvm/Kconfig
+++ b/arch/powerpc/kvm/Kconfig
@@ -30,7 +30,6 @@ config KVM_BOOK3S_HANDLER
 config KVM_BOOK3S_32_HANDLER
 	bool
 	select KVM_BOOK3S_HANDLER
-	select KVM_MMIO
 
 config KVM_BOOK3S_64_HANDLER
 	bool
@@ -38,7 +37,6 @@ config KVM_BOOK3S_64_HANDLER
 
 config KVM_BOOK3S_PR_POSSIBLE
 	bool
-	select KVM_MMIO
 
 config KVM_BOOK3S_HV_POSSIBLE
 	bool
@@ -201,7 +199,6 @@ config KVM_E500V2
 	depends on PPC_E500 && !PPC_E500MC
 	depends on !CONTEXT_TRACKING_USER
 	select KVM
-	select KVM_MMIO
 	help
 	  Support running unmodified E500 guest kernels in virtual machines on
 	  E500v2 host processors.
@@ -216,7 +213,6 @@ config KVM_E500MC
 	depends on PPC_E500MC
 	depends on !CONTEXT_TRACKING_USER
 	select KVM
-	select KVM_MMIO
 	select KVM_BOOKE_HV
 	help
 	  Support running unmodified E500MC/E5500/E6500 guest kernels in
diff --git a/arch/riscv/kvm/Kconfig b/arch/riscv/kvm/Kconfig
index ec2cee0a39e01..3aea8d4939d2e 100644
--- a/arch/riscv/kvm/Kconfig
+++ b/arch/riscv/kvm/Kconfig
@@ -28,7 +28,6 @@ config KVM
 	select KVM_COMMON
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
 	select KVM_GENERIC_HARDWARE_ENABLING
-	select KVM_MMIO
 	select VIRT_XFER_TO_GUEST_WORK
 	select SCHED_INFO
 	select GUEST_PERF_EVENTS if PERF_EVENTS
diff --git a/arch/s390/include/asm/kvm_host_s390.h b/arch/s390/include/asm/kvm_host_s390.h
index cd692f8fb7643..3c65c52d6682b 100644
--- a/arch/s390/include/asm/kvm_host_s390.h
+++ b/arch/s390/include/asm/kvm_host_s390.h
@@ -34,6 +34,8 @@
 
 #define KVM_S390_MANAGES_S390_GUEST 1
 
+#define __KVM_NO_MMIO
+
 /*
  * These seem to be used for allocating ->chip in the routing table, which we
  * don't use. 1 is as small as we can get to reduce the needed memory. If we
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 538ed1e803326..48ca567b0a846 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -32,7 +32,6 @@ config KVM_X86
 	select VHOST_TASK
 	select KVM_ASYNC_PF
 	select USER_RETURN_NOTIFIER
-	select KVM_MMIO
 	select SCHED_INFO
 	select PERF_EVENTS
 	select GUEST_PERF_EVENTS
diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index 03bfc92864b6e..dc9b68793f0d7 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -833,7 +833,7 @@ struct kvm {
 	struct kvm_vm_stat stat;
 	struct kvm_arch arch;
 	refcount_t users_count;
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 	struct kvm_coalesced_mmio_ring *coalesced_mmio_ring;
 	spinlock_t ring_lock;
 	struct list_head coalesced_zones;
diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
index c3c0ee253fc77..c012f30127e38 100644
--- a/virt/kvm/Kconfig
+++ b/virt/kvm/Kconfig
@@ -40,9 +40,6 @@ config NEED_KVM_DIRTY_RING_WITH_BITMAP
 	bool
 	depends on HAVE_KVM_DIRTY_RING
 
-config KVM_MMIO
-       bool
-
 config KVM_ASYNC_PF
        bool
 
diff --git a/virt/kvm/Makefile.kvm b/virt/kvm/Makefile.kvm
index d047d4cf58c9f..c4fb1e01d0c89 100644
--- a/virt/kvm/Makefile.kvm
+++ b/virt/kvm/Makefile.kvm
@@ -5,9 +5,8 @@
 
 KVM ?= ../../../virt/kvm
 
-kvm-y := $(KVM)/kvm_main.o $(KVM)/eventfd.o $(KVM)/binary_stats.o
+kvm-y := $(KVM)/kvm_main.o $(KVM)/eventfd.o $(KVM)/coalesced_mmio.o $(KVM)/binary_stats.o
 kvm-$(CONFIG_KVM_VFIO) += $(KVM)/vfio.o
-kvm-$(CONFIG_KVM_MMIO) += $(KVM)/coalesced_mmio.o
 kvm-$(CONFIG_KVM_ASYNC_PF) += $(KVM)/async_pf.o
 kvm-$(CONFIG_HAVE_KVM_IRQ_ROUTING) += $(KVM)/irqchip.o
 kvm-$(CONFIG_HAVE_KVM_DIRTY_RING) += $(KVM)/dirty_ring.o
diff --git a/virt/kvm/coalesced_mmio.c b/virt/kvm/coalesced_mmio.c
index 6b1d901610998..6bd4157b3ef41 100644
--- a/virt/kvm/coalesced_mmio.c
+++ b/virt/kvm/coalesced_mmio.c
@@ -15,6 +15,8 @@
 #include <linux/slab.h>
 #include <linux/kvm.h>
 
+#ifndef __KVM_NO_MMIO
+
 #include "coalesced_mmio.h"
 
 static inline struct kvm_coalesced_mmio_dev *to_mmio(struct kvm_io_device *dev)
@@ -188,3 +190,4 @@ int kvm_vm_ioctl_unregister_coalesced_mmio(struct kvm *kvm,
 	 */
 	return 0;
 }
+#endif
diff --git a/virt/kvm/coalesced_mmio.h b/virt/kvm/coalesced_mmio.h
index 36f84264ed257..77b811dee27c2 100644
--- a/virt/kvm/coalesced_mmio.h
+++ b/virt/kvm/coalesced_mmio.h
@@ -11,7 +11,7 @@
  *
  */
 
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 
 #include <linux/list.h>
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 65eb26a0520d8..2f25b75dbfd44 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4059,7 +4059,7 @@ static vm_fault_t kvm_vcpu_fault(struct vm_fault *vmf)
 	else if (vmf->pgoff == KVM_PIO_PAGE_OFFSET)
 		page = virt_to_page(vcpu->arch.pio_data);
 #endif
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 	else if (vmf->pgoff == KVM_COALESCED_MMIO_PAGE_OFFSET)
 		page = virt_to_page(vcpu->kvm->coalesced_mmio_ring);
 #endif
@@ -4900,7 +4900,7 @@ static int kvm_vm_ioctl_check_extension_generic(struct kvm *kvm, long arg)
 	case KVM_CAP_ENABLE_CAP_VM:
 	case KVM_CAP_HALT_POLL:
 		return 1;
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 	case KVM_CAP_COALESCED_MMIO:
 		return KVM_COALESCED_MMIO_PAGE_OFFSET;
 	case KVM_CAP_COALESCED_PIO:
@@ -5241,7 +5241,7 @@ static long kvm_vm_ioctl(struct file *filp,
 		break;
 	}
 #endif
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 	case KVM_REGISTER_COALESCED_MMIO: {
 		struct kvm_coalesced_mmio_zone zone;
 
@@ -5563,7 +5563,7 @@ static long kvm_dev_ioctl(struct file *filp,
 #ifdef CONFIG_X86
 		r += PAGE_SIZE;    /* pio data page */
 #endif
-#ifdef CONFIG_KVM_MMIO
+#ifndef __KVM_NO_MMIO
 		r += PAGE_SIZE;    /* coalesced mmio ring page */
 #endif
 		break;

-- 
Jazz isn't dead. It just smells funny.