linux-kernel - [PATCH v4 5/5] KVM: arm64: Expose new KVM cap for cacheable PFNMAP

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Message-ID: <20250518054754.5345-6-ankita@nvidia.com>
Date: Sun, 18 May 2025 05:47:54 +0000
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Subject: [PATCH v4 5/5] KVM: arm64: Expose new KVM cap for cacheable PFNMAP

From: Ankit Agrawal <ankita@...dia.com>

Introduce a new KVM capability to expose to the userspace whether
cacheable mapping of PFNMAP is supported.

The ability to safely do the cacheable mapping of PFNMAP is contingent
on S2FWB and ARM64_HAS_CACHE_DIC. S2FWB allows KVM to avoid flushing
the D cache, ARM64_HAS_CACHE_DIC allows KVM to avoid flushing the icache
and turns icache_inval_pou() into a NOP. The cap would be false if
those requirements are missing.

This capability would allow userspace to discover the support.
This would be used in conjunction with the
KVM_MEM_ENABLE_CACHEABLE_PFNMAP memslot flag. Userspace is
required to query this capability before it can set the memslot
flag.

This cap could also be used by userspace to prevent live-migration
across FWB and non-FWB hosts.

CC: Catalin Marinas <catalin.marinas@....com>
CC: Jason Gunthorpe <jgg@...dia.com>
CC: Oliver Upton <oliver.upton@...ux.dev>
Suggested-by: Marc Zyngier <maz@...nel.org>
Signed-off-by: Ankit Agrawal <ankita@...dia.com>
---
 Documentation/virt/kvm/api.rst | 17 ++++++++++++++++-
 arch/arm64/kvm/arm.c           |  8 ++++++++
 include/uapi/linux/kvm.h       |  1 +
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 47c7c3f92314..ad4c5e131977 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8478,7 +8478,7 @@ ENOSYS for the others.
 When enabled, KVM will exit to userspace with KVM_EXIT_SYSTEM_EVENT of
 type KVM_SYSTEM_EVENT_SUSPEND to process the guest suspend request.
 
-7.37 KVM_CAP_ARM_WRITABLE_IMP_ID_REGS
+7.42 KVM_CAP_ARM_WRITABLE_IMP_ID_REGS
 -------------------------------------
 
 :Architectures: arm64
@@ -8496,6 +8496,21 @@ aforementioned registers before the first KVM_RUN. These registers are VM
 scoped, meaning that the same set of values are presented on all vCPUs in a
 given VM.
 
+7.43 KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED
+-------------------------------------------
+
+:Architectures: arm64
+:Target: VM
+:Parameters: None
+
+This capability indicate to the userspace whether a PFNMAP memory region
+can be safely mapped as cacheable. This relies on the presence of
+force write back (FWB) feature support on the hardware.
+
+The usermode could query this capability and subsequently set the
+KVM_MEM_ENABLE_CACHEABLE_PFNMAP memslot flag forming a handshake to
+activate the code.
+
 8. Other capabilities.
 ======================
 
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 68fec8c95fee..7855f579fb82 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -402,6 +402,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_SUPPORTED_REG_MASK_RANGES:
 		r = BIT(0);
 		break;
+	case KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED:
+		if (!kvm)
+			r = -EINVAL;
+		else
+			r = stage2_has_fwb(kvm->arch.mmu.pgt) &&
+			    cpus_have_final_cap(ARM64_HAS_CACHE_DIC);
+		break;
+
 	default:
 		r = 0;
 	}
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 9defefe7bdf0..fb868586d73d 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -931,6 +931,7 @@ struct kvm_enable_cap {
 #define KVM_CAP_X86_APIC_BUS_CYCLES_NS 237
 #define KVM_CAP_X86_GUEST_MODE 238
 #define KVM_CAP_ARM_WRITABLE_IMP_ID_REGS 239
+#define KVM_CAP_ARM_CACHEABLE_PFNMAP_SUPPORTED 240
 
 struct kvm_irq_routing_irqchip {
 	__u32 irqchip;
-- 
2.34.1