From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 1D6BF1DF27D for ; Thu, 25 Sep 2025 19:32:30 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758828752; cv=none; b=nh2c8Wk73i0uEsQgNnECcjTvXtaSqVRD5easfgc6m+CNGFGzWOxYKc7JUTNEE2kgB9Ewf6VVqVAVZLBRP0OukBACb0sm3inOiVfzs6rWodrfGsIv2ZlRYuYiHkKaWPcB4cE3dDgav/exeKjT8XrCdY6UWlHyZn+gjLcA39sCWFU= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1758828752; c=relaxed/simple; bh=jpqmsgmAQIqXP57KiX7lxY1y974aUYb01QrMbrU8Ahg=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=PI9QugTOWCU+EyuUjjec1913xXcfSp40SHw31xm28lYZo+jDVnXW0bp9mEGMjyGtvOu7Q+L7m9q4K7odUpD/Os3nAyqqzhNxjQInHtK81rVgsZLK4U662vSjWeUofipC5zAgnOyIGWUJZP/jMCJ/VRR36g+QhpOonEo0bkU2haU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=VPC/q4Ir; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="VPC/q4Ir" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1758828750; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=D5jCgvyWVjXaOxE5qhMkjeDubZJSnlT/2ztleqWrFCE=; b=VPC/q4IrPAbIk/S46tksLxrLnh+d4t1igIOUTnIkjkm1PqFaDWFzp1/cdg97WN9HiS5xIy oRH9o59Ikn+Xna/Ve+8YJaD7bKKDNzv0jXmhQXPTYCbFOY1AlaMuliAsHb38IZ74l5v8dB K2cezcHf6Syjr5OaxFqmX69Y+2Ys0SE= Received: from mail-wm1-f69.google.com (mail-wm1-f69.google.com [209.85.128.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-391-3oUzQ9ZmPqyrqBTdnHs2Og-1; Thu, 25 Sep 2025 15:32:28 -0400 X-MC-Unique: 3oUzQ9ZmPqyrqBTdnHs2Og-1 X-Mimecast-MFC-AGG-ID: 3oUzQ9ZmPqyrqBTdnHs2Og_1758828747 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-46e377d8c80so4350815e9.3 for ; Thu, 25 Sep 2025 12:32:28 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1758828747; x=1759433547; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:x-gm-message-state:from:to:cc:subject:date:message-id :reply-to; bh=D5jCgvyWVjXaOxE5qhMkjeDubZJSnlT/2ztleqWrFCE=; b=GmldF9VVsunx+FMAJu9dK8CoLWlyWh5IZB+5BWpLOhG4OdrCZPU7ujJDM3kOBqTk1u t6QYMdKIeBWV9+zrPYcxybFqFELqQGHRrrxZ+qnrVDUEOZKVuNeMv0zu8vA9bvuIG9si oMNlHTNEUzTaOdwN8b3id4Q5i5Est5kffGgGYcKZGwDBF4OCRCSMNIeJyX9j2+9vG8KE jtgq+7o2NFRvV2DQAhcpay/GL2sCNfEMkYlb5Mr+NSP8eh9x85EMyJB10tFZDqxdtGTY LRa0qJ8NNkjna6G3jQphX0k8y7HpKBbZtfCm5hSPIO/TV4eM1SQjZj/TnIJb4x4k/PWT J8Ww== X-Forwarded-Encrypted: i=1; AJvYcCU7+mWhset275uBtUL79lbB/sdkygTSJAuFwMYHzFcmodnRQYvGdL+t3C/70Nn1L9boOJY+IEj/m3rTkW5C@vger.kernel.org X-Gm-Message-State: AOJu0YwrOH9IjYKDIvOBkj07UIgi2GZNBgTAnatEIaHaJWR7byGXTEhD Qrq0OAK49tqGncKtf30jPJq1akNyUyUsxeyqQFWO3pyY0sxYSvcAXmlrRhdUnez3ssVE4My3yq9 MoTk8LsAqGVnUK+Rs3l87yLVghEunP/AaIMW2AZoFL4s8aEdGF6b5zml2s8PDUgKfa7A= X-Gm-Gg: ASbGnct6VTc3q/nODBv9r15PjoQ7kGPEp1JfFsmmLBEAgGkfrY+9pPi/BZ+3BQuSKnh IlwTxVxzMjxvAZCmgxWoiZG6iVZoaf1/Q/TDQEbAvapxogKHO1P541kPjkIU430V+VSoj9c+IAy huFYZ/n44zvOdbjgL9XLju2LYJpMfkil0ihBkoPy/EH6658y4dELOI49XXiwsDnhrUzq33hsY08 vlN4gQ/0y3beb5tB0BFZ6Z1QZakCcGlKyrYFMU8gARNFqZJeHUr4eFRjNLAynzuPbWx44dAeJ45 iDFJeYSx2uTWWVtIlcA2CgKnq4OjyvKA900zCCZ21phE4Hh+DP9qSr9CyebLBWG3Zfe+pTXRe6U dWQZzRMgsyQE1U9JevWaXmqUmzUIBhkPbwz9QhyQ4t27ngGk3qTBlctdLvVoXuIhDzYNb X-Received: by 2002:a05:6000:3102:b0:3f9:1571:fe04 with SMTP id ffacd0b85a97d-40e4b1a06b4mr4993180f8f.48.1758828747340; Thu, 25 Sep 2025 12:32:27 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEFUL795NU1gcu1A2U+hWPDoMkyU660Ad1UyOUZxyr2NlakU+3syyUj7XJkeC88am/EkGOIvw== X-Received: by 2002:a05:6000:3102:b0:3f9:1571:fe04 with SMTP id ffacd0b85a97d-40e4b1a06b4mr4993145f8f.48.1758828746777; Thu, 25 Sep 2025 12:32:26 -0700 (PDT) Received: from ?IPV6:2003:d8:2f3f:f800:c101:5c9f:3bc9:3d08? (p200300d82f3ff800c1015c9f3bc93d08.dip0.t-ipconnect.de. [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 Precedence: bulk X-Mailing-List: linux-fsdevel@vger.kernel.org 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: "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 Content-Language: en-US Autocrypt: addr=david@redhat.com; keydata= xsFNBFXLn5EBEAC+zYvAFJxCBY9Tr1xZgcESmxVNI/0ffzE/ZQOiHJl6mGkmA1R7/uUpiCjJ dBrn+lhhOYjjNefFQou6478faXE6o2AhmebqT4KiQoUQFV4R7y1KMEKoSyy8hQaK1umALTdL QZLQMzNE74ap+GDK0wnacPQFpcG1AE9RMq3aeErY5tujekBS32jfC/7AnH7I0v1v1TbbK3Gp XNeiN4QroO+5qaSr0ID2sz5jtBLRb15RMre27E1ImpaIv2Jw8NJgW0k/D1RyKCwaTsgRdwuK Kx/Y91XuSBdz0uOyU/S8kM1+ag0wvsGlpBVxRR/xw/E8M7TEwuCZQArqqTCmkG6HGcXFT0V9 PXFNNgV5jXMQRwU0O/ztJIQqsE5LsUomE//bLwzj9IVsaQpKDqW6TAPjcdBDPLHvriq7kGjt WhVhdl0qEYB8lkBEU7V2Yb+SYhmhpDrti9Fq1EsmhiHSkxJcGREoMK/63r9WLZYI3+4W2rAc UucZa4OT27U5ZISjNg3Ev0rxU5UH2/pT4wJCfxwocmqaRr6UYmrtZmND89X0KigoFD/XSeVv jwBRNjPAubK9/k5NoRrYqztM9W6sJqrH8+UWZ1Idd/DdmogJh0gNC0+N42Za9yBRURfIdKSb B3JfpUqcWwE7vUaYrHG1nw54pLUoPG6sAA7Mehl3nd4pZUALHwARAQABzSREYXZpZCBIaWxk ZW5icmFuZCA8ZGF2aWRAcmVkaGF0LmNvbT7CwZoEEwEIAEQCGwMCF4ACGQEFCwkIBwICIgIG FQoJCAsCBBYCAwECHgcWIQQb2cqtc1xMOkYN/MpN3hD3AP+DWgUCaJzangUJJlgIpAAKCRBN 3hD3AP+DWhAxD/9wcL0A+2rtaAmutaKTfxhTP0b4AAp1r/eLxjrbfbCCmh4pqzBhmSX/4z11 opn2KqcOsueRF1t2ENLOWzQu3Roiny2HOU7DajqB4dm1BVMaXQya5ae2ghzlJN9SIoopTWlR 0Af3hPj5E2PYvQhlcqeoehKlBo9rROJv/rjmr2x0yOM8qeTroH/ZzNlCtJ56AsE6Tvl+r7cW 3x7/Jq5WvWeudKrhFh7/yQ7eRvHCjd9bBrZTlgAfiHmX9AnCCPRPpNGNedV9Yty2Jnxhfmbv Pw37LA/jef8zlCDyUh2KCU1xVEOWqg15o1RtTyGV1nXV2O/mfuQJud5vIgzBvHhypc3p6VZJ lEf8YmT+Ol5P7SfCs5/uGdWUYQEMqOlg6w9R4Pe8d+mk8KGvfE9/zTwGg0nRgKqlQXrWRERv cuEwQbridlPAoQHrFWtwpgYMXx2TaZ3sihcIPo9uU5eBs0rf4mOERY75SK+Ekayv2ucTfjxr Kf014py2aoRJHuvy85ee/zIyLmve5hngZTTe3Wg3TInT9UTFzTPhItam6dZ1xqdTGHZYGU0O otRHcwLGt470grdiob6PfVTXoHlBvkWRadMhSuG4RORCDpq89vu5QralFNIf3EysNohoFy2A LYg2/D53xbU/aa4DDzBb5b1Rkg/udO1gZocVQWrDh6I2K3+cCs7BTQRVy5+RARAA59fefSDR 9nMGCb9LbMX+TFAoIQo/wgP5XPyzLYakO+94GrgfZjfhdaxPXMsl2+o8jhp/hlIzG56taNdt VZtPp3ih1AgbR8rHgXw1xwOpuAd5lE1qNd54ndHuADO9a9A0vPimIes78Hi1/yy+ZEEvRkHk /kDa6F3AtTc1m4rbbOk2fiKzzsE9YXweFjQvl9p+AMw6qd/iC4lUk9g0+FQXNdRs+o4o6Qvy iOQJfGQ4UcBuOy1IrkJrd8qq5jet1fcM2j4QvsW8CLDWZS1L7kZ5gT5EycMKxUWb8LuRjxzZ 3QY1aQH2kkzn6acigU3HLtgFyV1gBNV44ehjgvJpRY2cC8VhanTx0dZ9mj1YKIky5N+C0f21 zvntBqcxV0+3p8MrxRRcgEtDZNav+xAoT3G0W4SahAaUTWXpsZoOecwtxi74CyneQNPTDjNg azHmvpdBVEfj7k3p4dmJp5i0U66Onmf6mMFpArvBRSMOKU9DlAzMi4IvhiNWjKVaIE2Se9BY FdKVAJaZq85P2y20ZBd08ILnKcj7XKZkLU5FkoA0udEBvQ0f9QLNyyy3DZMCQWcwRuj1m73D sq8DEFBdZ5eEkj1dCyx+t/ga6x2rHyc8Sl86oK1tvAkwBNsfKou3v+jP/l14a7DGBvrmlYjO 59o3t6inu6H7pt7OL6u6BQj7DoMAEQEAAcLBfAQYAQgAJgIbDBYhBBvZyq1zXEw6Rg38yk3e EPcA/4NaBQJonNqrBQkmWAihAAoJEE3eEPcA/4NaKtMQALAJ8PzprBEXbXcEXwDKQu+P/vts IfUb1UNMfMV76BicGa5NCZnJNQASDP/+bFg6O3gx5NbhHHPeaWz/VxlOmYHokHodOvtL0WCC 8A5PEP8tOk6029Z+J+xUcMrJClNVFpzVvOpb1lCbhjwAV465Hy+NUSbbUiRxdzNQtLtgZzOV Zw7jxUCs4UUZLQTCuBpFgb15bBxYZ/BL9MbzxPxvfUQIPbnzQMcqtpUs21CMK2PdfCh5c4gS sDci6D5/ZIBw94UQWmGpM/O1ilGXde2ZzzGYl64glmccD8e87OnEgKnH3FbnJnT4iJchtSvx yJNi1+t0+qDti4m88+/9IuPqCKb6Stl+s2dnLtJNrjXBGJtsQG/sRpqsJz5x1/2nPJSRMsx9 5YfqbdrJSOFXDzZ8/r82HgQEtUvlSXNaXCa95ez0UkOG7+bDm2b3s0XahBQeLVCH0mw3RAQg r7xDAYKIrAwfHHmMTnBQDPJwVqxJjVNr7yBic4yfzVWGCGNE4DnOW0vcIeoyhy9vnIa3w1uZ 3iyY2Nsd7JxfKu1PRhCGwXzRw5TlfEsoRI7V9A8isUCoqE2Dzh3FvYHVeX4Us+bRL/oqareJ CIFqgYMyvHj7Q06kTKmauOe4Nf0l0qEkIuIzfoLJ3qr5UyXc2hLtWyT9Ir+lYlX9efqh7mOY qIws/H2t In-Reply-To: <20250925155051.2959-1-roypat@amazon.co.uk> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit 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