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"john.fastabend@gmail.com" , "kpsingh@kernel.org" , "sdf@fomichev.me" , "haoluo@google.com" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "peterx@redhat.com" , "jannh@google.com" , "pfalcato@suse.de" , "shuah@kernel.org" , "seanjc@google.com" , "kvm@vger.kernel.org" , "linux-doc@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "kvmarm@lists.linux.dev" , "linux-fsdevel@vger.kernel.org" , "linux-mm@kvack.org" , "bpf@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "Cali, Marco" , "Kalyazin, Nikita" , "Thomson, Jack" , "derekmn@amazon.co.uk" , "tabba@google.com" , "ackerleytng@google.com" Subject: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing Thread-Topic: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing Thread-Index: AQHcLWcRbocNw3xUv0GREpQeD3nXQA== Date: Wed, 24 Sep 2025 15:22:40 +0000 Message-ID: <20250924152214.7292-3-roypat@amazon.co.uk> References: 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version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Add an option to not perform TLB flushes after direct map manipulations.=0A= TLB flushes result in a up to 40x elongation of page faults in=0A= guest_memfd (scaling with the number of CPU cores), or a 5x elongation=0A= of memory population, which is inacceptable when wanting to use direct=0A= map removed guest_memfd as a drop-in replacement for existing workloads.=0A= =0A= TLB flushes are not needed for functional correctness (the virt->phys=0A= mapping technically stays "correct", the kernel should simply not use it=0A= for a while), so we can skip them to keep performance in-line with=0A= "traditional" VMs.=0A= =0A= Enabling this option means that the desired protection from=0A= Spectre-style attacks is not perfect, as an attacker could try to=0A= prevent a stale TLB entry from getting evicted, keeping it alive until=0A= the page it refers to is used by the guest for some sensitive data, and=0A= then targeting it using a spectre-gadget.=0A= =0A= Cc: Will Deacon =0A= Signed-off-by: Patrick Roy =0A= ---=0A= include/linux/kvm_host.h | 1 +=0A= virt/kvm/guest_memfd.c | 3 ++-=0A= virt/kvm/kvm_main.c | 3 +++=0A= 3 files changed, 6 insertions(+), 1 deletion(-)=0A= =0A= diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h=0A= index 73a15cade54a..4d2bc18860fc 100644=0A= --- a/include/linux/kvm_host.h=0A= +++ b/include/linux/kvm_host.h=0A= @@ -2298,6 +2298,7 @@ extern unsigned int halt_poll_ns;=0A= extern unsigned int halt_poll_ns_grow;=0A= extern unsigned int halt_poll_ns_grow_start;=0A= extern unsigned int halt_poll_ns_shrink;=0A= +extern bool guest_memfd_tlb_flush;=0A= =0A= struct kvm_device {=0A= const struct kvm_device_ops *ops;=0A= diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c=0A= index b7129c4868c5..d8dd24459f0d 100644=0A= --- a/virt/kvm/guest_memfd.c=0A= +++ b/virt/kvm/guest_memfd.c=0A= @@ -63,7 +63,8 @@ static int kvm_gmem_folio_zap_direct_map(struct folio *fo= lio)=0A= if (!r) {=0A= unsigned long addr =3D (unsigned long) folio_address(folio);=0A= folio->private =3D (void *) ((u64) folio->private & KVM_GMEM_FOLIO_NO_DI= RECT_MAP);=0A= - flush_tlb_kernel_range(addr, addr + folio_size(folio));=0A= + if (guest_memfd_tlb_flush)=0A= + flush_tlb_kernel_range(addr, addr + folio_size(folio));=0A= }=0A= =0A= return r;=0A= diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c=0A= index b5e702d95230..753c06ebba7f 100644=0A= --- a/virt/kvm/kvm_main.c=0A= +++ b/virt/kvm/kvm_main.c=0A= @@ -95,6 +95,9 @@ unsigned int halt_poll_ns_shrink =3D 2;=0A= module_param(halt_poll_ns_shrink, uint, 0644);=0A= EXPORT_SYMBOL_GPL(halt_poll_ns_shrink);=0A= =0A= +bool guest_memfd_tlb_flush =3D true;=0A= +module_param(guest_memfd_tlb_flush, bool, 0444);=0A= +=0A= /*=0A= * Allow direct access (from KVM or the CPU) without MMU notifier protecti= on=0A= * to unpinned pages.=0A= -- =0A= 2.51.0=0A= =0A=