Re: [PATCH] kvm/x86: add capability to disable the write-track mechanism

[Sean Christopherson <seanjc@google.com>]

On Mon, Feb 05, 2024, Andrei Vagin wrote:
> 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.
> 
> This change introduces the new KVM_CAP_PAGE_WRITE_TRACKING capability,
> allowing users to enable/disable the write-track mechanism. It is enabled
> by default for backward compatibility.

I would much prefer to allocate the write-tracking metadata on-demand in
kvm_page_track_register_notifier(), i.e. do the same as mmu_first_shadow_root_alloc(),
except for just gfn_write_track.

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.

That way there's no need for new uAPI and no need for userspace changes.