Re: [PATCH v2] KVM: Ignore MMU notifiers for guest_memfd-only memslots

["Lorenzo Stoakes (ARM)" <ljs@kernel.org>]

On Thu, Nov 05, 2026 at 11:02:28AM -0800, Sean Christopherson wrote:
> On Wed, Nov 04, 2026, Lorenzo Stoakes (ARM) wrote:
> > [updated David's email address to @kernel.org]
> >
> > On Wed, Aug 12, 2026 at 10:42:50AM +0100, Alexandru Elisei wrote:
> > [ ... ]
> > > On Fri, Jul 24, 2026 at 12:52:20PM -0700, Sean Christopherson wrote:
> > > > It'll probably take me a few weeks to circle back to this, as I finally crawled
> > > > out from underneath a pile of urgent and am just now getting back into review
> > > > mode, but this is fairly high on my todo list, so with luck we might still squeeze
> > > > this into 7.3.
> >
> > Hey Sean,
> >
> > I wondered whether you'd had a chance to look at this?
> >
> > After discussing this with Marc and Alex it is clear that this is really a
> > blocker for Alex's SPE series, as we need a sensible means of ensuring
> > that stage-2 mappings stay in place.
> >
> > So ideally would prefer to get something like this in in the next cycle,
> > if you don't have any objections to it? :)
> >
> > If it'd be easier, I think a smaller version of the same patch would also
> > give us what we need? I.e. your snippet from June with the page ageing part
> > dropped, as below.
>
> The GPC invalidation fix landed, so we can go straight to the super simple v1.
> Compile tested only at this point, I'll formally post this (hopefully today),
> assuming nothing explodes in testing.

Perfect, thanks very much :)

>
> --
> From: Alexandru Elisei <alexandru.elisei@arm.com>
> Date: Mon, 15 Jun 2026 16:52:44 +0100
> Subject: [PATCH] KVM: Skip gfn-based MMU notifier invalidations for
>  guest_memfd-only memslots
>
> Don't invalidate gmem-only memslots when responding to MMU notifier
> invalidations, as the entire point of gmem-only memslots is so that KVM
> doesn't need a VMA to install mappings into KVM's MMU.  Specificially,
> page faults are resolved by going straight to guest_memfd, using the gfn
> and the associated memslot's gmem.file.  I.e. whether or not userspace
> happens to have valid mappings to the same gmem instance is irrelevant,
> as are any invalidations for those mappings.
>
> Note!  The exception is KVM's gfn_to_pfn_cache implementation which isn't
> yet wired up to pull pfns directly from guest_memfd, i.e. still relies on
> userspace mappings/VMAs and thus need to be invalidated, e.g. to ensure KVM
> doesn't retain a kernel mapping+pointer to PRIVATE memory.  However, as of
> commit da260bcb56fe ("KVM: pfncache: use a dedicated invalidation sequence
> for cache refresh"), GPC invalidation no longer relies on memslot lookups,
> because GPCs also support HVA-based mappings.  I.e. KVM *must* be able to
> detect and handle relevant MMU notifier invalidations even if there is *no*
> memslot whatsoever.
>
> In addition to the obvious performance benefits, avoiding unnecessary and
> spurious SPTE zaps and TLB flushes will allow KVM to support features like
> ARM's Statistical Profiling Extension (SPE)[1] and x86's Secure AVIC[2],
> both of which rely on the pages *never* being unmapped from the stage-2
> page tables.
>
> Link: https://lore.kernel.org/all/20260903160623.315525-1-alexandru.elisei@arm.com [1]
> Link: https://lore.kernel.org/all/cover.1783490022.git.naveen@kernel.org [2]
> Cc: Lorenzo Stoakes (ARM) <ljs@kernel.org>
> Cc: Naveen N Rao (AMD) <naveen@kernel.org>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
> [sean: rewrite changelog to capture the new gpc behavior]
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  virt/kvm/kvm_main.c | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index cf4cb586f140..599dea22afb8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -601,6 +601,10 @@ static __always_inline kvm_mn_ret_t kvm_handle_hva_range(struct kvm *kvm,
>  			unsigned long hva_start, hva_end;
>
>  			slot = container_of(node, struct kvm_memory_slot, hva_node[slots->node_idx]);
> +
> +			if (kvm_slot_has_gmem(slot) && kvm_memslot_is_gmem_only(slot))
> +				continue;
> +
>  			hva_start = max_t(unsigned long, range->start, slot->userspace_addr);
>  			hva_end = min_t(unsigned long, range->end,
>  					slot->userspace_addr + (slot->npages << PAGE_SHIFT));
>
> base-commit: d21eef2084a9af87ac2a783bc7c2b175ba114e4e
> --

--
Cheers, Lorenzo