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[2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-40fb72facf9sm2650164f8f.13.2025.09.25.04.02.51 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Sep 2025 04:02:53 -0700 (PDT) Message-ID: Date: Thu, 25 Sep 2025 13:02:51 +0200 Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v7 06/12] KVM: guest_memfd: add module param for disabling TLB flushing To: "Roy, Patrick" Cc: "pbonzini@redhat.com" , "corbet@lwn.net" , "maz@kernel.org" , "oliver.upton@linux.dev" , "joey.gouly@arm.com" , "suzuki.poulose@arm.com" , "yuzenghui@huawei.com" , "catalin.marinas@arm.com" , "will@kernel.org" , "tglx@linutronix.de" , "mingo@redhat.com" , "bp@alien8.de" , "dave.hansen@linux.intel.com" , "x86@kernel.org" , "hpa@zytor.com" , "luto@kernel.org" , "peterz@infradead.org" , "willy@infradead.org" , "akpm@linux-foundation.org" , "lorenzo.stoakes@oracle.com" , "Liam.Howlett@oracle.com" , "vbabka@suse.cz" , "rppt@kernel.org" , "surenb@google.com" , "mhocko@suse.com" , "song@kernel.org" , "jolsa@kernel.org" , "ast@kernel.org" , "daniel@iogearbox.net" , "andrii@kernel.org" , "martin.lau@linux.dev" , "eddyz87@gmail.com" , "yonghong.song@linux.dev" , "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" References: <20250924151101.2225820-4-patrick.roy@campus.lmu.de> <20250924152214.7292-1-roypat@amazon.co.uk> <20250924152214.7292-3-roypat@amazon.co.uk> From: David Hildenbrand Autocrypt: addr=david@redhat.com; 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charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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 > Signed-off-by: Patrick Roy > --- > 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