kvm/x86: allocate the write-tracking metadata on-demand

The write-track is used externally only by the gpu/drm/i915 driver.
Currently, it is always enabled, if a kernel has been compiled with this
driver.

Enabling the write-track mechanism adds a two-byte overhead per page across
all memory slots. It isn't significant for regular VMs. However in gVisor,
where the entire process virtual address space is mapped into the VM, even
with a 39-bit address space, the overhead amounts to 256MB.

Rework the write-tracking mechanism to enable it on-demand in
kvm_page_track_register_notifier.

Here is Sean's comment about the locking scheme:

The only potential hiccup would be if taking slots_arch_lock would
deadlock, but it should be impossible for slots_arch_lock to be taken in
any other path that involves VFIO and/or KVMGT *and* can be coincident.
Except for kvm_arch_destroy_vm() (which deletes KVM's internal
memslots), slots_arch_lock is taken only through KVM ioctls(), and the
caller of kvm_page_track_register_notifier() *must* hold a reference to
the VM.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Andrei Vagin <avagin@google.com>
Link: https://lore.kernel.org/r/20240213192340.2023366-1-avagin@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index b5b2d0fde579..7d33a2605ad5 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1466,6 +1466,15 @@ struct kvm_arch {
 	 */
 	bool shadow_root_allocated;
 
+#ifdef CONFIG_KVM_EXTERNAL_WRITE_TRACKING
+	/*
+	 * If set, the VM has (or had) an external write tracking user, and
+	 * thus all write tracking metadata has been allocated, even if KVM
+	 * itself isn't using write tracking.
+	 */
+	bool external_write_tracking_enabled;
+#endif
+
 #if IS_ENABLED(CONFIG_HYPERV)
 	hpa_t	hv_root_tdp;
 	spinlock_t hv_root_tdp_lock;