Re: [PATCH v7 04/42] KVM: Stub in ability to disable per-VM memory attribute tracking

[Sean Christopherson <seanjc@google.com>]

On Fri, May 22, 2026, Ackerley Tng wrote:
> From: Sean Christopherson <seanjc@google.com>
> 
> Introduce the basic infrastructure to allow per-VM memory attribute
> tracking to be disabled. This will be built-upon in a later patch, where a
> module param can disable per-VM memory attribute tracking.
> 
> Split the Kconfig option into a base KVM_MEMORY_ATTRIBUTES and the
> existing KVM_VM_MEMORY_ATTRIBUTES. The base option provides the core
> plumbing, while the latter enables the full per-VM tracking via an xarray
> and the associated ioctls.
> 
> kvm_get_memory_attributes() now performs a static call that either looks up
> kvm->mem_attr_array with CONFIG_KVM_VM_MEMORY_ATTRIBUTES is enabled, or
> just returns 0 otherwise. The static call can be patched depending on
> whether per-VM tracking is enabled by the CONFIG.
> 
> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Fuad Tabba <tabba@google.com>
> Signed-off-by: Ackerley Tng <ackerleytng@google.com>
> ---

...

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index abb9cfa3eb04d..ee26f1d9b5fda 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -101,6 +101,17 @@ EXPORT_SYMBOL_FOR_KVM_INTERNAL(halt_poll_ns_shrink);
>  static bool __ro_after_init allow_unsafe_mappings;
>  module_param(allow_unsafe_mappings, bool, 0444);
>  
> +#ifdef CONFIG_KVM_MEMORY_ATTRIBUTES
> +#ifdef CONFIG_KVM_VM_MEMORY_ATTRIBUTES
> +static bool vm_memory_attributes = true;
> +#else
> +#define vm_memory_attributes false
> +#endif
> +DEFINE_STATIC_CALL_RET0(__kvm_get_memory_attributes, kvm_get_memory_attributes_t);
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(STATIC_CALL_KEY(__kvm_get_memory_attributes));
> +EXPORT_SYMBOL_FOR_KVM_INTERNAL(STATIC_CALL_TRAMP(__kvm_get_memory_attributes));
> +#endif

Fudge.  This morning's PUCK discussion about VBS made me realize that we really
don't want to kill off _all_ per-VM attributes like this, we really just want to
kill off PRIVATE.  And even if RWX protections never arrive, conceptually shoving
all attributes into guest_memfd doesn't make any sense, because it really is only
the private vs. shared state that is tied to the physical memory, things like RWX
protections aren't so tightly couple to the data.

It'll require a bit of minor surgery to these patches, but the silver lining is
that I think the end code will be slightly easier to follow.

I'll sync with you off-list to splice in the changes to your current series (I
have them sketched out).