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Howlett" , "Miguel Ojeda" , "Boqun Feng" , =?utf-8?q?Bj=C3=B6rn_Roy_Baron?= , "Benno Lossin" , "Trevor Gross" , "Danilo Krummrich" Cc: , , Subject: Re: [PATCH] rust: page: add volatile memory copy methods X-Mailer: aerc 0.21.0 References: <20260130-page-volatile-io-v1-1-19f3d3e8f265@kernel.org> <877bszrz37.fsf@t14s.mail-host-address-is-not-set> In-Reply-To: <877bszrz37.fsf@t14s.mail-host-address-is-not-set> X-ClientProxiedBy: LO4P265CA0026.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:2ae::23) To LOVP265MB8871.GBRP265.PROD.OUTLOOK.COM (2603:10a6:600:488::16) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-MS-PublicTrafficType: Email X-MS-TrafficTypeDiagnostic: LOVP265MB8871:EE_|LO3P265MB2044:EE_ X-MS-Office365-Filtering-Correlation-Id: 85b79647-034c-4e8f-ab20-08de6009e585 X-MS-Exchange-SenderADCheck: 1 X-MS-Exchange-AntiSpam-Relay: 0 X-Microsoft-Antispam: BCL:0;ARA:13230040|1800799024|366016|7416014|376014|7053199007|921020; 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Copying data to/from `Pa= ge` >>> from/to these buffers would be undefined behavior if regular memcpy >>> operations are used. >>> >>> The operation can be made well defined, if the buffers that potentially >>> observe racy operations can be said to exist outside of any Rust >>> allocation. For this to be true, the kernel must only interact with the >>> buffers using raw volatile reads and writes. >>> >>> Add methods on `Page` to read and write the contents using volatile >>> operations. >>> >>> Also improve clarity by specifying additional requirements on >>> `read_raw`/`write_raw` methods regarding concurrent operations on invol= ved >>> buffers. >>> >>> Signed-off-by: Andreas Hindborg >>> --- >>> rust/kernel/page.rs | 53 +++++++++++++++++++++++++++++++++++++++++++++= ++++++++ >>> 1 file changed, 53 insertions(+) >>> >>> diff --git a/rust/kernel/page.rs b/rust/kernel/page.rs >>> index 432fc0297d4a8..6568a0d3b3baa 100644 >>> --- a/rust/kernel/page.rs >>> +++ b/rust/kernel/page.rs >>> @@ -7,6 +7,7 @@ >>> bindings, >>> error::code::*, >>> error::Result, >>> + ffi::c_void, >>> uaccess::UserSliceReader, >>> }; >>> use core::{ >>> @@ -260,6 +261,8 @@ fn with_pointer_into_page( >>> /// # Safety >>> /// >>> /// * Callers must ensure that `dst` is valid for writing `len` by= tes. >>> + /// * Callers must ensure that there are no other concurrent reads= or writes to/from the >>> + /// destination memory region. >>> /// * Callers must ensure that this call does not race with a writ= e to the same page that >>> /// overlaps with this read. >>> pub unsafe fn read_raw(&self, dst: *mut u8, offset: usize, len: us= ize) -> Result { >>> @@ -274,6 +277,30 @@ pub unsafe fn read_raw(&self, dst: *mut u8, offset= : usize, len: usize) -> Result >>> }) >>> } >>> >>> + /// Maps the page and reads from it into the given IO memory regio= n using volatile memory >>> + /// operations. >>> + /// >>> + /// This method will perform bounds checks on the page offset. If = `offset .. offset+len` goes >>> + /// outside of the page, then this call returns [`EINVAL`]. >>> + /// >>> + /// # Safety >>> + /// Callers must ensure that: >>> + /// >>> + /// * The destination memory region is outside of any Rust memory = allocation. >>> + /// * The destination memory region is writable. >>> + /// * This call does not race with a write to the same source page= that overlaps with this read. >>> + pub unsafe fn read_raw_toio(&self, dst: *mut u8, offset: usize, le= n: usize) -> Result { >>> + self.with_pointer_into_page(offset, len, move |src| { >>> + // SAFETY: If `with_pointer_into_page` calls into this clo= sure, then >>> + // it has performed a bounds check and guarantees that `sr= c` is >>> + // valid for `len` bytes. >>> + // >>> + // There caller guarantees that there is no data race at t= he source. >>> + unsafe { bindings::memcpy_toio(dst.cast::(), src.c= ast::(), len) }; >> >> I feel that this should be a generic utility that integrates with our IO= infra >> that allows you to copy/from IO to a slice. > > While that might also be useful, for my particular use case I am copying > between two pages. One is mapped from user space, the other one is > allocated by a driver. No slices involved. Pasting for reference [1]: Then what you need is a byte-wise atomic memcpy, not memcpy_{from,to}io. Best, Gary > > > /// Copy data to the current page of this segment from `src_page`. > /// > /// Copies `PAGE_SIZE - (self.offset() % PAGE_SIZE` bytes of data fr= om `src_page` to this > /// segment starting at `self.offset()` from offset `self.offset() % = PAGE_SIZE`. This call > /// will advance offset and reduce length of `self`. > /// > /// Returns the number of bytes copied. > pub fn copy_from_page(&mut self, src_page: &Page, src_offset: usize) = -> usize { > let dst_offset =3D self.offset() % PAGE_SIZE; > debug_assert!(src_offset <=3D PAGE_SIZE); > let length =3D (PAGE_SIZE - dst_offset) > .min(self.len() as usize) > .min(PAGE_SIZE - src_offset); > let page_idx =3D self.offset() / PAGE_SIZE; > > // SAFETY: self.bio_vec is valid and thus bv_page must be a valid > // pointer to a `struct page`. > let dst_page =3D unsafe { Page::from_raw(self.bio_vec.bv_page.add= (page_idx)) }; > > dst_page > .with_pointer_into_page(dst_offset, length, |dst| { > // SAFETY: > // - If `with_pointer_into_page` calls this closure, then= it has performed bounds > // checks and guarantees that `dst` is valid for `lengt= h` bytes. > // - Since we have a shared reference to `src_page`, the = read cannot race with any > // writes to `src_page`. > unsafe { src_page.read_raw_toio(dst, src_offset, length) = } > }) > .expect("Assertion failure, bounds check failed."); > > self.advance(length as u32) > .expect("Assertion failure, bounds check failed."); > > length > > > > Best regards, > Andreas Hindborg > > [1] https://github.com/metaspace/linux/blob/3e46e95f0707fa71259b1d241f689= 144ad61cc62/rust/kernel/block/bio/vec.rs#L142