Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing

David Hildenbrand
2025-09-25T11:02:59Z

On 24.09.25 17:22, Roy, Patrick wrote:
> Add an option to not perform TLB flushes after direct map manipulations.
> TLB flushes result in a up to 40x elongation of page faults in
> guest_memfd (scaling with the number of CPU cores), or a 5x elongation
> of memory population, which is inacceptable when wanting to use direct
> map removed guest_memfd as a drop-in replacement for existing workloads.
>
> TLB flushes are not needed for functional correctness (the virt->phys
> mapping technically stays "correct", the kernel should simply not use it
> for a while), so we can skip them to keep performance in-line with
> "traditional" VMs.
>
> Enabling this option means that the desired protection from
> Spectre-style attacks is not perfect, as an attacker could try to
> prevent a stale TLB entry from getting evicted, keeping it alive until
> the page it refers to is used by the guest for some sensitive data, and
> then targeting it using a spectre-gadget.
>
> Cc: Will Deacon <will@kernel.org>
> Signed-off-by: Patrick Roy <roypat@amazon.co.uk>
> ---
> include/linux/kvm_host.h | 1 +
> virt/kvm/guest_memfd.c | 3 ++-
> virt/kvm/kvm_main.c | 3 +++
> 3 files changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 73a15cade54a..4d2bc18860fc 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2298,6 +2298,7 @@ extern unsigned int halt_poll_ns;
> extern unsigned int halt_poll_ns_grow;
> extern unsigned int halt_poll_ns_grow_start;
> extern unsigned int halt_poll_ns_shrink;
> +extern bool guest_memfd_tlb_flush;
>
> struct kvm_device {
> const struct kvm_device_ops *ops;
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index b7129c4868c5..d8dd24459f0d 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -63,7 +63,8 @@ static int kvm_gmem_folio_zap_direct_map(struct folio *folio)
> if (!r) {
> unsigned long addr = (unsigned long) folio_address(folio);
> folio->private = (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DIRECT_MAP);
> - flush_tlb_kernel_range(addr, addr + folio_size(folio));
> + if (guest_memfd_tlb_flush)
> + flush_tlb_kernel_range(addr, addr + folio_size(folio));
> }
>
> return r;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index b5e702d95230..753c06ebba7f 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -95,6 +95,9 @@ unsigned int halt_poll_ns_shrink = 2;
> module_param(halt_poll_ns_shrink, uint, 0644);
> EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);
>
> +bool guest_memfd_tlb_flush = true;
> +module_param(guest_memfd_tlb_flush, bool, 0444);

The parameter name is a bit too generic. I think you somehow have to
incorporate the "direct_map" aspects.

Also, I wonder if this could be a capability per vm/guest_memfd?

Then, you could also nicely document the semantics, considerations,
impact etc :)

--
Cheers

David / dhildenb