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 1BBD42F39C8 for ; Fri, 10 Oct 2025 14:08:22 +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=1760105304; cv=none; b=J8S2S6wB3X1mFv/e3NTeR0YodLOj8vDWta3Wv0MwJ2f3m1YefdAQhCUAH17JH6wr62u6q76T2RZhHIS3iu66qWfDDQ5rv5tJXN6Ppi23pd0oc/J0wBb3oL7LDSnprtsqvjIhFzgSwo4woVoZtZ7zbOqiBw1vqKsnJLy/pO5SEDY= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1760105304; c=relaxed/simple; bh=lL8xuqSu+d7lDRb0p7j++/rm9tvUYUilMwM1JJtICvM=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=lHVA3RBZEzLrdmr9GGpwWWpgD2/lVjB0smN8iMpR1tK68llOAGYUcuxkapdL11y0kzaGyl9eXyLBVmdXbkVH17xU9F/ydrXmWhu+JidXVz7XQ098zlkSL4egJ5tqYWLE/ePKgOxZ6BG9iwiht1HBI9yjHd4fYy8avFEfTAo/d3w= 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=f2wj237u; 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="f2wj237u" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1760105302; 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=PYVEMHVdJ49+bxbOS3d3XXISG+vWJV0m5a54YxhfpOg=; b=f2wj237uiut5WnkHNiTg6G6jnWF5sFzKmiIU8HAYxD3gM/iK+MWrDb8e+bNg8RjA6d9zfU MZghd7HAfiUa7u6wHbw5SGghuBDwX2I1BKLAA7ZqIGZcIf01fMxM5K0vIbK2wGZY4LB0SN oSQZnkVPAGqSBO2FxRTu3Nqz8YKETQk= 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-537-uYtGetHjPkqKFVjMYCWfbA-1; Fri, 10 Oct 2025 10:08:20 -0400 X-MC-Unique: uYtGetHjPkqKFVjMYCWfbA-1 X-Mimecast-MFC-AGG-ID: uYtGetHjPkqKFVjMYCWfbA_1760105300 Received: by mail-wm1-f70.google.com with SMTP id 5b1f17b1804b1-46e502a37cdso13727955e9.0 for ; Fri, 10 Oct 2025 07:08:20 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1760105299; x=1760710099; 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=PYVEMHVdJ49+bxbOS3d3XXISG+vWJV0m5a54YxhfpOg=; b=naWnnPspIRs/EyUtoWZfVRMD85ObnEN1csPKpSEJNqONresNLmskpQ9UxE2USCMoEz moCgKwsIyDq7AYIfFwzcHVERxQF77A6EIIMidcSHQEiOkHksfllob6ft2Eq7T4Bfiqdz /y+FqJwW5Y8o3/jA3l2IiJI4DgxOD/PnUIrfjPERcSWtaUp3heUZDcTqLF2wr0cnDcS0 7GjgE6/w6zm6Y0YYTkQza8W4Pdb2Gs13X9YmOy3wnDGRitiVvjLmUEcbMnH8dTLkCvQ4 QWnt8v2cI/MzNGoeHjMUlh50F25vyfkSuUGnH6lFhhX2e61jazEmavXc1REDLJCWZCJ+ qycg== X-Forwarded-Encrypted: i=1; AJvYcCUVbUB5nTJylvdo4PliyVwy/vsIExJSQlz2GxD9Ea3gd32qLq/sbmQENTeIUVpicF3Mn3bQ1lBSQLmshHw=@vger.kernel.org X-Gm-Message-State: AOJu0YyyZque1pChluWIRVWLvg5w2A1qlMQhG36dMXThAphL2kx+dLnu 27v5hM7sKc2GfNiDEQzL5ZtnlRFV6S4nqAyUv3RsEiQqrjFjHWYBMFfz84KH3/6ub7LErP+c9I2 sJgpuMChMqIArMES5BEFOHzRCXlc3c9wefDNJ3Hc3iZZi5laRvre4lr5DSRzR0nKAHA== X-Gm-Gg: ASbGncsLoaYTX8I/X9flkuBkX4Gqlv1bPc4ObIRyvA+g9bgApWebIfEDajziEL1tj+P 9urcfRX/o/3gN532aCzqq4opwlTYqMe3KBKpeewRU8UlOIFmPzHIIvfCOM62ppewJwedS+d7B90 ujo0OzoKES/WgaYYx7KvR2dsSnBfhDPZoietGtC984xMHmMhLlJgmFu2uaVyEokC8XtgIesmvIx Pl6YLil6c0Aj+KbNVAu8BtRIRR6jOMHSgnoIOyOd3MUtX2iV5POBimAIUPSXvtGftcEOwNceoP2 WuGauZJ6eZ8OAm0VOcvOCI3YyVaGuyo2433mKXBw1z+HVt6A0n28whiLAkU8Oi6Cfy1J1Jd8sAn vTJE= X-Received: by 2002:a05:600c:3d87:b0:46e:50ce:a353 with SMTP id 5b1f17b1804b1-46fa9aa1c5cmr83128205e9.14.1760105299513; Fri, 10 Oct 2025 07:08:19 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHlIHObVAPABnAu5EcY0tKLPigJXyaX44aIR5uNy5jIyRCvW4JMawATPpYqyH9oEC6tab319A== X-Received: by 2002:a05:600c:3d87:b0:46e:50ce:a353 with SMTP id 5b1f17b1804b1-46fa9aa1c5cmr83127975e9.14.1760105299037; Fri, 10 Oct 2025 07:08:19 -0700 (PDT) Received: from [192.168.3.141] (tmo-083-189.customers.d1-online.com. [80.187.83.189]) by smtp.gmail.com with ESMTPSA id 5b1f17b1804b1-46fb4989601sm46900725e9.9.2025.10.10.07.08.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 10 Oct 2025 07:08:18 -0700 (PDT) Message-ID: <06d4d0a5-5cc7-4554-b9bc-7d160bf7a50f@redhat.com> Date: Fri, 10 Oct 2025 16:08:16 +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 v2 02/13] KVM: guest_memfd: Add INIT_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: <20251003232606.4070510-1-seanjc@google.com> <20251003232606.4070510-3-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: <20251003232606.4070510-3-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 04.10.25 01:25, Sean Christopherson wrote: > Add a guest_memfd flag to allow userspace to state that the underlying > memory should be configured to be initialized as shared, and reject user > page faults if the guest_memfd instance's memory isn't shared. Because > KVM doesn't yet support in-place private<=>shared conversions, all > guest_memfd memory effectively follows the initial state. > > Alternatively, KVM could deduce the initial 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. > > For x86 CoCo VMs, which don't yet support MMAP, memory is currently private > by default (otherwise the memory would be unusable). If MMAP implies > memory is shared by default, then the default state for CoCo VMs will vary > based on MMAP, and from userspace's perspective, will change when in-place > conversion support is added. I.e. to maintain guest<=>host ABI, userspace > would need to immediately convert all memory from shared=>private, which > is both ugly and inefficient. The inefficiency could be avoided by adding > a flag to state that memory is _private_ by default, irrespective of MMAP, > but that would lead to an equally messy and hard to document ABI. > > Bite the bullet and immediately add a flag to control the default state so > that the effective behavior is explicit and straightforward. > > Fixes: 3d3a04fad25a ("KVM: Allow and advertise support for host mmap() on guest_memfd files") > Cc: David Hildenbrand > Reviewed-by: Fuad Tabba > Tested-by: Fuad Tabba > Reviewed-by: Ackerley Tng > Signed-off-by: Sean Christopherson > --- Reviewed-by: David Hildenbrand -- Cheers David / dhildenb