From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 07185C282EC for ; Thu, 6 Mar 2025 10:46:53 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1tq8jm-0002nC-Vc; Thu, 06 Mar 2025 05:46:01 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tq8jf-0002hG-Hg for qemu-devel@nongnu.org; Thu, 06 Mar 2025 05:45:52 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1tq8jY-0001Kw-Q8 for qemu-devel@nongnu.org; Thu, 06 Mar 2025 05:45:50 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1741257942; 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=2RljU9Zr10QDqicNQECmrUW4OwRQHr/PVy0Eh7KJRi8=; b=aExG40T9xQDGxI72mNYr50JdOvq4uPKQVjMkv1+8FcnUS10yZIQOr2QCg3wLx6/Fif7aBY XY9Dg0jda0uJSoeXRVMXuS/BBrQ4gnWOrOPO+XCJE9kkBQi1TcIsYRf5mQr8CBeecJgmjC 2u/4zTl6Jg2rZEUgqkD64TgjGvmkHKY= 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-467-mmUUXYxGO_yv0qsmlY-zeA-1; Thu, 06 Mar 2025 05:45:25 -0500 X-MC-Unique: mmUUXYxGO_yv0qsmlY-zeA-1 X-Mimecast-MFC-AGG-ID: mmUUXYxGO_yv0qsmlY-zeA_1741257924 Received: by mail-wm1-f69.google.com with SMTP id 5b1f17b1804b1-43bd0586a73so4878795e9.2 for ; Thu, 06 Mar 2025 02:45:24 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1741257924; x=1741862724; 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=2RljU9Zr10QDqicNQECmrUW4OwRQHr/PVy0Eh7KJRi8=; b=A0XsieuKcKYEyDPH+xVNNaTBAjCx3xS3Gkr2JOYMA0+0IQnV8ICq44fBaiOsct11/E FeL1yK6FIZkoQAFFtqOTtFO4Xq0j1G+A759ngX3kD6EaKNgBUN33aJWY8hINxygPwA/A o1Hi0iqyXj1HBClWrjxcRbKD+u+NIBJwc7rXqRGx+D8dw0JMuL/JWeWNxl0fNpMZGAlb fY29Ccc10BM21aYWQjNpipW2HK8T5D+IAZWgB10ot/x396XdL/umJdZf01QHrJA0rUzu K/uKwER4huOhrBZaSsrKuo6OpOsGTmFDI6Kcbbf6tzOdF8MdSu/HztTX59uyKp7iHsNE SSag== X-Gm-Message-State: AOJu0YyHgHw/BmR2Aut4mnDgPqiyU4IEUNH48uWg4xLuc5scsGMyHzGy JKMbjxiXHNjwoICDQ5ugrn62rBN3sFZUdbMiewLjGDG43pCzQzkK2wBi9iqP9PAOBMNPda+zLN4 sE5x24y1rDVUDaD9LjEg7qSqHBhuujFs62T6R6lANMbWhi0FrYCWX X-Gm-Gg: ASbGncuMn11H3eIBxnQ9g6sydzChOnfQxs+7IZrjpgFuDnW7+a8tytolyS+6CYEaPeE i672MjHfBNemWfaTDxU1SyEDtd6iKZqC/XvnxRCUxo/SCDzzD7CWfKhqWyTkTfrodNj9/Faf5I/ TCXzI/+RH/ug7hrlR+xwDWNFqMNVTiFUS2hmKZO4qgwto+THXN7ck23uSWup4QGQz0EJkRrnjWZ cI5rk+pfHM6cSxVRd/oCMbJ6Vp+59dVdS6mkbjUY8rGcr/0FNY9eQp4cjcBo49k6epZI8rN+1Zy IMswjDPoIpVdL95kRyQ= X-Received: by 2002:a05:600c:4f11:b0:439:916a:b3db with SMTP id 5b1f17b1804b1-43bd292fee0mr62130745e9.6.1741257923720; Thu, 06 Mar 2025 02:45:23 -0800 (PST) X-Google-Smtp-Source: AGHT+IEx6S3GRGnDVxfuUVsSupILL7H65IFBkgNm5JXCL3nK9b7LVnTBjD8g4+tbWGrdoYTDocXg6A== X-Received: by 2002:a05:600c:4f11:b0:439:916a:b3db with SMTP id 5b1f17b1804b1-43bd292fee0mr62130535e9.6.1741257923342; Thu, 06 Mar 2025 02:45:23 -0800 (PST) Received: from [192.168.10.48] ([151.95.119.44]) by smtp.googlemail.com with ESMTPSA id 5b1f17b1804b1-43bd42c6203sm46087855e9.24.2025.03.06.02.45.22 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 06 Mar 2025 02:45:22 -0800 (PST) Message-ID: <90a21eff-a7b4-41dd-99df-c36fd83a1c08@redhat.com> Date: Thu, 6 Mar 2025 11:45:19 +0100 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 04/12] rust: timer: wrap QEMUTimer with Opaque<> and express pinning requirements To: Zhao Liu Cc: qemu-devel@nongnu.org, qemu-rust@nongnu.org References: <20250227142219.812270-1-pbonzini@redhat.com> <20250227142219.812270-5-pbonzini@redhat.com> <7b09b4e3-3c1f-4c94-ad3a-054eaf74f24c@redhat.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Received-SPF: pass client-ip=170.10.129.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.001, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On 3/4/25 10:13, Zhao Liu wrote: >> - const fn as_mut_ptr(&self) -> *mut Self { >> - self as *const Timer as *mut _ >> + /// Create a `Timer` struct without initializing it. >> + /// >> + /// # Safety >> + /// >> + /// The timer must be initialized before it is armed with >> + /// [`modify`](Self::modify). >> + pub unsafe fn new() -> Self { >> + // SAFETY: requirements relayed to callers of Timer::new >> + Self(unsafe { Opaque::zeroed() }) > > We should use Opaque::uninit()? Because MaybeUninit::::zeroed() > marks the timer as initialized, which disables MaybeUninit's ability to check for > initialization. e.g., While neither is good, a zeroed area of memory behaves better than an uninitialized one... In particular, Drop calls timer_del() which works fine with a zeroed QEMUTimer. With Opaque::uninit() you could have a crash just with drop(Timer::new()); > // No compiling error or runtime panic > let t: MaybeUninit = MaybeUninit::zeroed(); > let _t = unsafe { t.assume_init() }; > > Further more, I spent some time trying to figure out if MaybeUninit in > Opaque<> could help identify UB caused by uninitialized Timer, but I found > it doesn't work. :-( > > // No compiling error or runtime panic > let mut v: UnsafeCell> = UnsafeCell::new(MaybeUninit::uninit()); > let _v = unsafe { v.get_mut().assume_init() }; > > But when I adjust MaybeUninit as the outer wrapper, the UB check can > work: > > // Runtime panic: Illegal instruction > let v: MaybeUninit> = MaybeUninit::uninit(); > let _v = unsafe { v.assume_init() }; > > Compared with linux's Opaque, it also puts MaybeUninit on the outermost > layer. Yes, I admit I just copied what Linux does. :) > And there's another example: > > https://doc.rust-lang.org/std/cell/struct.UnsafeCell.html#method.raw_get > > Emm, I guess now we have UnsafeCell> because interior > mutability is expected... but this layout breaks MaybeUninit's functionality. Thanks for the example from the documentation! Indeed it should be possible to do /// Returns a raw mutable pointer to the opaque data. pub const fn as_mut_ptr(&self) -> *mut T { UnsafeCell::raw_get(self.value.as_ptr()) } /// Returns a raw pointer to the opaque data that can be passed to a /// C function as `void *`. pub const fn as_void_ptr(&self) -> *mut std::ffi::c_void { self.as_mut_ptr().cast() } pub const fn raw_get(slot: *const Self) -> *mut T { // SAFETY: even if uninitialized, slot points to a MaybeUninit let slot = slot.cast::>>; UnsafeCell::raw_get(slot.as_ptr()) } if Opaque<> uses a MaybeUninit>. I'm a bit worried of deviating from what Linux does though... Paolo