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

[Andrei Vagin]

On Mon, Feb 5, 2024 at 10:41 AM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> 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.

I think it is a good idea, I don't know why I didn't consider it.
Thanks. I will prepare a new patch.

Thanks,
Andrei