From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 9F0B721B918 for ; Mon, 29 Sep 2025 08:38:54 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759135136; cv=none; b=WePmtdv5NCYgboRigc+pJn+Io1x7e575VAQjeS6nH7Xz5h7IfPqXKRf2RnmoXm0KuZQZnFVxHiOJgjnVJ0pBqucd4QzH5eX31U6Sa+sayxjBtEsDAy0cSraJIvVhNDuV86JH9clVX6rjvAG6Ix86gJHauLkz+2Fc28K+j9n3yX8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1759135136; c=relaxed/simple; bh=DGXaWOvvKM7o4hekCoZdxdQgjfWoeYeqiar+JIXO8BI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=jjvA6Qffp7vsQ4+a0CC/vbc3Rk+9SspahQELsqBeiPM/s9qLMQNY40HEDk3zK3K0HA3j/ex0R9Yxddx5ytUKLtSxNUMgOlFVkiVcdqXEk8+ksrNUlYoIADTgjlTRfR+1ht6F2L09BTdb4vReVbJz5UvvVcPndE4JfOWJFxmQjYc= 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=Z87CiL/a; arc=none smtp.client-ip=170.10.133.124 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="Z87CiL/a" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1759135133; 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=AsaMWIYtbBtVBfpmpyiR+SeKnf95HfK7yUC9+oaG6D8=; b=Z87CiL/aVJ17tVsAAp8K9zqnYIVRwfvkGIiKNyww7oNZy1Q145xtFme+80zcqrTvZdQwv9 219HrwadnirYhPZ02KwTAHtZ0okEwkBxVYIk2ORYUBpU0kzbyZ8KMsAQ6gFLN53uFm3+iB qUJpIc/B6zQTeb3TFyEPROlYPkZiwvs= Received: from mail-wm1-f70.google.com (mail-wm1-f70.google.com [209.85.128.70]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-536-UnxVUgf0OZCoswggS_XP9g-1; Mon, 29 Sep 2025 04:38:51 -0400 X-MC-Unique: UnxVUgf0OZCoswggS_XP9g-1 X-Mimecast-MFC-AGG-ID: UnxVUgf0OZCoswggS_XP9g_1759135131 Received: by mail-wm1-f70.google.com with SMTP id 5b1f17b1804b1-46e377d8c80so15057195e9.3 for ; Mon, 29 Sep 2025 01:38:51 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1759135130; x=1759739930; 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=AsaMWIYtbBtVBfpmpyiR+SeKnf95HfK7yUC9+oaG6D8=; b=evyNWgYMlOGIkG9cHmmq1Bws/Db/zD3MH5UJOSak7Ub5QqYKbDKYXGqZoyDA6FN6Ii Kj+kTMXFKanbU6TkCQhqyaUnJduO28Gil+O/rdqNDEYOEcVdyK0e0ow189jaam3drj9q K+i0yHAyuQKzsAN05kHrjdSdSYWSBdtNQ1E2ZCnIG7wPCIrUUBWFVFevnmOwlwIsrAq7 2tAGDrOUeKW8GOl9aCLNFgdTRJQ1SBhnLIh0HBtDZ3z9luv43Gd+uAUfz85f03maPr/X zBwbq9W9x7ZKUKS1iEZEjBsD2NMERiTS/SfViYKMvLiORDFP27qUhp/Lh6xWSYzZMkQ+ FJTw== X-Forwarded-Encrypted: i=1; AJvYcCWJlot+EwSnOJ+0ACOIdXy+VqA9YN2fILIUt9x/r6Yz4ttFdAd4fUVojX5mfMrnizgaIs634MkKbIekSQo=@vger.kernel.org X-Gm-Message-State: AOJu0YxSzXC6pnmMYpm9+8/EJeMaG/ZEuDLWmDUDIgraLDEPTvAfQCZ9 2tMVmNXNzVD2+shPmN1Y9I/qrWlkZ5OsEwkVIJceqtT/2c83SDWsLWaFc9N3oqUE04lvSW+C98F wXSls4QLFdscWH8yqjnabU58KCn/5KQqkBTF6KrqJetcFe/WjKPi0UgX8haBSnRuigw== X-Gm-Gg: ASbGnctIwDn8Wwq+S7YsuA60L7potLqQf9bGAMbnpwSCDNeWLR0F2tLJ8qUhk2dxu7D 5ZlJwK9q7zMj23AfGEuSLAvWi9ZfCqOHVtRUgS+WgZHsjG4oXcUzxsDT2aIbs2lPVc6Rb+2LYHL qt1noN34I1rt0/PsITSPc+MSAEYnelyAZwK0ygfKzV5l7UsOYc+ATiXiHEt1UtfidJ99V6yIUdQ WJQ7zJQcHZKTS155gRw9VkZaXUSEtCqLjk8LsqZWbvxmB396SiXFWeJMToFm8vj+tmRwuEtsxii 1+zhFhNeIZLx1rKOUp7X4DVLGGhCVsWu0BilC+9bFNKnInDi999JO46Wmui8DCsJxpQT9V4KdZ6 4NKRsYMR3vOFX6PuW7BZP2BNoBEuAmKai/yRvkScA1+y6SB435fhGgxqU43iYcgElRg== X-Received: by 2002:a05:600c:630f:b0:46e:1c16:7f42 with SMTP id 5b1f17b1804b1-46e329aec2dmr153480975e9.11.1759135130580; Mon, 29 Sep 2025 01:38:50 -0700 (PDT) X-Google-Smtp-Source: AGHT+IEcV6BjsdOdU1GfaihKJ08y3WjEiB6j1KPpJKZ3FGWr7FHfxfJvDumut+aa2UJzUHnPY07gaw== X-Received: by 2002:a05:600c:630f:b0:46e:1c16:7f42 with SMTP id 5b1f17b1804b1-46e329aec2dmr153480715e9.11.1759135130200; Mon, 29 Sep 2025 01:38:50 -0700 (PDT) Received: from ?IPV6:2003:d8:2f05:e100:526f:9b8:bd2a:2997? (p200300d82f05e100526f09b8bd2a2997.dip0.t-ipconnect.de. [2003:d8:2f05:e100:526f:9b8:bd2a:2997]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46e4c048f73sm56989435e9.24.2025.09.29.01.38.49 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 29 Sep 2025 01:38:49 -0700 (PDT) Message-ID: <7ce29e23-aea9-4d4d-b686-3b7a752e0276@redhat.com> Date: Mon, 29 Sep 2025 10:38:48 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/6] KVM: guest_memfd: Add DEFAULT_SHARED flag, reject user page faults if not set To: Sean Christopherson , Paolo Bonzini , Christian Borntraeger , Janosch Frank , Claudio Imbrenda Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Fuad Tabba , Ackerley Tng References: <20250926163114.2626257-1-seanjc@google.com> <20250926163114.2626257-2-seanjc@google.com> 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: <20250926163114.2626257-2-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit On 26.09.25 18:31, Sean Christopherson wrote: > Add a guest_memfd flag to allow userspace to state that the underlying > memory should be configured to be shared by default, and reject user page > faults if the guest_memfd instance's memory isn't shared by default. > Because KVM doesn't yet support in-place private<=>shared conversions, all > guest_memfd memory effectively follows the default state. I recall we discussed exactly that in the past (e.g., on April 17) in the call: "Current plan: * guest_memfd creation flag to specify “all memory starts as shared” * Compatible with the old behavior where all memory started as private * Initially, only these can be mmap (no in-place conversion) " > > Alternatively, KVM could deduce the default state based on MMAP, which for > all intents and purposes is what KVM currently does. However, implicitly > deriving the default state based on MMAP will result in a messy ABI when > support for in-place conversions is added. I don't recall the details, but I faintly remember that we discussed later that with mmap support, the default will be shared for now, and that no other flag would be required for the time being. We could always add a "DEFAULT_PRIVATE" flag when we realize that we would have to change the default later. Ackerley might remember more details. -- Cheers David / dhildenb