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[2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-410f2007372sm3193903f8f.16.2025.09.25.12.32.24 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 25 Sep 2025 12:32:26 -0700 (PDT) Message-ID: <1b34dd89-cbbb-474e-be75-808e174c8ed9@redhat.com> Date: Thu, 25 Sep 2025 21:32:22 +0200 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: "Liam.Howlett@oracle.com" , "ackerleytng@google.com" , "akpm@linux-foundation.org" , "andrii@kernel.org" , "ast@kernel.org" , "bp@alien8.de" , "bpf@vger.kernel.org" , "catalin.marinas@arm.com" , "corbet@lwn.net" , "daniel@iogearbox.net" , "dave.hansen@linux.intel.com" , "derekmn@amazon.co.uk" , "eddyz87@gmail.com" , "haoluo@google.com" , "hpa@zytor.com" , "Thomson, Jack" , "jannh@google.com" , "jgg@ziepe.ca" , "jhubbard@nvidia.com" , "joey.gouly@arm.com" , "john.fastabend@gmail.com" , "jolsa@kernel.org" , "Kalyazin, Nikita" , "kpsingh@kernel.org" , "kvm@vger.kernel.org" , "kvmarm@lists.linux.dev" , "linux-arm-kernel@lists.infradead.org" , "linux-doc@vger.kernel.org" , "linux-fsdevel@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-kselftest@vger.kernel.org" , "linux-mm@kvack.org" , "lorenzo.stoakes@oracle.com" , "luto@kernel.org" , "martin.lau@linux.dev" , "maz@kernel.org" , "mhocko@suse.com" , "mingo@redhat.com" , "oliver.upton@linux.dev" , "pbonzini@redhat.com" , "peterx@redhat.com" , "peterz@infradead.org" , "pfalcato@suse.de" , "rppt@kernel.org" , "sdf@fomichev.me" , "seanjc@google.com" , "shuah@kernel.org" , "song@kernel.org" , "surenb@google.com" , "suzuki.poulose@arm.com" , "tabba@google.com" , "tglx@linutronix.de" , "vbabka@suse.cz" , "will@kernel.org" , "willy@infradead.org" , "x86@kernel.org" , "Cali, Marco" , "yonghong.song@linux.dev" , "yuzenghui@huawei.com" References: <20250925155051.2959-1-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 X-Rspam-User: X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 6A976120008 X-Stat-Signature: 8s5dqhqnxopdf3t5e616tmsysqa5tdc5 X-HE-Tag: 1758828751-96382 X-HE-Meta: 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 nXynmIXc 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 25.09.25 17:50, Roy, Patrick wrote: > On Thu, 2025-09-25 at 12:02 +0100, David Hildenbrand wrote: >> 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. > > Fair :) > >> Also, I wonder if this could be a capability per vm/guest_memfd? > > I don't really have any opinions on how to expose this knob, but I > thought capabilities should be additive? (e.g. we only have > KVM_ENABLE_EXTENSION(), and then having a capability with a negative > polarity "enable to _not_ do TLB flushes" is a bit weird in my head). Well, you are enabling the "skip-tlbflush" feature :) So a kernel that knows that extension could skip tlb flushes. So I wouldn't see this as "perform-tlbflush" but "skip-tlbflush" / "no-tlbflush" > Then again, if people are fine having TLB flushes be opt-in instead of > opt-out (Will's comment on v6 makes me believe that the opt-out itself > might already be controversial for arm64), a capability would work. Yeah, I think this definitely should be opt-in: opt-in to slightly less security in a given timeframe by performing less tlb flushes. -- Cheers David / dhildenb