From: Ankit Agrawal <ankita@nvidia.com>

Introduce a new memslot flag KVM_MEM_ENABLE_CACHEABLE_PFNMAP
as a tool for userspace to indicate that it expects a particular
PFN range to be mapped cacheable.

This will serve as a guide for the KVM to activate the code that
allows cacheable PFNMAP.

CC: Oliver Upton <oliver.upton@linux.dev>
CC: Catalin Marinas <catalin.marinas@arm.com>
CC: Jason Gunthorpe <jgg@nvidia.com>
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
---
 include/uapi/linux/kvm.h | 1 +
 virt/kvm/kvm_main.c      | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index b6ae8ad8934b..9defefe7bdf0 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -51,6 +51,7 @@ struct kvm_userspace_memory_region2 {
 #define KVM_MEM_LOG_DIRTY_PAGES	(1UL << 0)
 #define KVM_MEM_READONLY	(1UL << 1)
 #define KVM_MEM_GUEST_MEMFD	(1UL << 2)
+#define KVM_MEM_ENABLE_CACHEABLE_PFNMAP	(1UL << 3)
 
 /* for KVM_IRQ_LINE */
 struct kvm_irq_level {
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index c7ecca504cdd..cddda7f21413 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1524,7 +1524,8 @@ static void kvm_replace_memslot(struct kvm *kvm,
  * only allows these.
  */
 #define KVM_SET_USER_MEMORY_REGION_V1_FLAGS \
-	(KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY)
+	(KVM_MEM_LOG_DIRTY_PAGES | KVM_MEM_READONLY | \
+	 KVM_MEM_ENABLE_CACHEABLE_PFNMAP)
 
 bool __weak kvm_arch_supports_cacheable_pfnmap(void)
 {
-- 
2.34.1

On Sat, May 24, 2025 at 01:39:41AM +0000, ankita@nvidia.com wrote:
> From: Ankit Agrawal <ankita@nvidia.com>
> 
> Introduce a new memslot flag KVM_MEM_ENABLE_CACHEABLE_PFNMAP
> as a tool for userspace to indicate that it expects a particular
> PFN range to be mapped cacheable.
> 
> This will serve as a guide for the KVM to activate the code that
> allows cacheable PFNMAP.
> 
> CC: Oliver Upton <oliver.upton@linux.dev>
> CC: Catalin Marinas <catalin.marinas@arm.com>
> CC: Jason Gunthorpe <jgg@nvidia.com>
> Suggested-by: Marc Zyngier <maz@kernel.org>
> Signed-off-by: Ankit Agrawal <ankita@nvidia.com>
> ---
>  include/uapi/linux/kvm.h | 1 +
>  virt/kvm/kvm_main.c      | 3 ++-
>  2 files changed, 3 insertions(+), 1 deletion(-)

I thought we agreed not to do this? Sean was strongly against it
right?

There is no easy way for VFIO to know to set it, and the kernel will
not allow switching a cachable VMA to non-cachable anyhow.

So all it does is make it harder to create a memslot.

Jason