From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from casper.infradead.org (casper.infradead.org [90.155.50.34]) (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 AE644239570; Mon, 14 Jul 2025 10:55:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=90.155.50.34 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752490527; cv=none; b=YTVK5IDXzWvZhOvR3RtPddYfRP/qDjlXf3ux7uoYJGD4iFu4UZk/h/2wlpkpo/5hEyK513KDN25WD5W8Ggikv++3j+VhNUpCOENYytU3I+BkYfRyFShhTtCJfjMhy4PPIfDN1fo5vMnNqotAAQihZw4F5PcWyEMaBP9Q+qxNQPA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752490527; c=relaxed/simple; bh=dYNW+G9IkWXMB5UjzS7cVgR6jqNbPLgCYskPD++uc54=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=Df+2tzMG3/nkiNIworvTYmzeefhO4KjafkM7TRoQLxpHrHE+ZAqJcMcwtTbwW0fTLqprwylZrK2uGXBgZ+DYS+pLIm5BiNaS/OkCK5sVRALtZEYHyQ/J7UgLxEWnWptIkgjM+fMmM9EguTriVz5HyIQKDF43aMKHLsBGT7nWsuw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org; spf=none smtp.mailfrom=infradead.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b=N4D6me88; arc=none smtp.client-ip=90.155.50.34 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: smtp.subspace.kernel.org; spf=none smtp.mailfrom=infradead.org Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="N4D6me88" DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=io1tVKXnDmOOI4XAw6Cl9XPcaBmlolImRCBWLa00h8I=; b=N4D6me884F1hBjIu5NxRO7FgvY iXLtliNvdrcSD8/DIyEzvNJYZBZABGiXogm2TnPRf7I5Y8QOT0/jzkf3/DIh+ST6w7305iOxhodEd mgJxBWZRB25kusp+aKjCUeyCmjROcjZG0LCGqPdCUB8+AzV1WkgyqgKUhTKjh1JXjF15Iom/z3g0L +9kNFg38iKHO3n4FUXUhs73cKTob67z5QgqWf3YnsUlL12FvR3DTPKzed82xsBEP4mS7POZKUYCUe R0T6pylJ8olwL3CKTaoviWWTUhiCMiCgd5yZm45AYEtrcJqWaoWvMRHXZ/6wPnQWdq0YGhZTxADi3 +CGUYysA==; Received: from 77-249-17-252.cable.dynamic.v4.ziggo.nl ([77.249.17.252] helo=noisy.programming.kicks-ass.net) by casper.infradead.org with esmtpsa (Exim 4.98.2 #2 (Red Hat Linux)) id 1ubGq9-00000006w5j-0q8W; Mon, 14 Jul 2025 10:55:21 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id BE084300186; Mon, 14 Jul 2025 12:55:20 +0200 (CEST) Date: Mon, 14 Jul 2025 12:55:20 +0200 From: Peter Zijlstra To: Ard Biesheuvel Cc: Ard Biesheuvel , linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Will Deacon , Mark Rutland , Sebastian Andrzej Siewior Subject: Re: [RFC PATCH 7/7] arm64/efi: Call EFI runtime services without disabling preemption Message-ID: <20250714105520.GS905792@noisy.programming.kicks-ass.net> References: <20250514174339.1834871-9-ardb+git@google.com> <20250514174339.1834871-16-ardb+git@google.com> <20250711134833.GI905792@noisy.programming.kicks-ass.net> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Mon, Jul 14, 2025 at 12:20:30PM +1000, Ard Biesheuvel wrote: > On Fri, 11 Jul 2025 at 23:48, Peter Zijlstra wrote: > > > > On Wed, May 14, 2025 at 07:43:47PM +0200, Ard Biesheuvel wrote: > > > From: Ard Biesheuvel > > > > > > The only remaining reason why EFI runtime services are invoked with > > > preemption disabled is the fact that the mm is swapped out behind the > > > back of the context switching code. > > > > > > The kernel no longer disables preemption in kernel_neon_begin(). > > > Furthermore, the EFI spec is being clarified to explicitly state that > > > only baseline FP/SIMD is permitted in EFI runtime service > > > implementations, and so the existing kernel mode NEON context switching > > > code is sufficient to preserve and restore the execution context of an > > > in-progress EFI runtime service call. > > > > > > Most EFI calls are made from the efi_rts_wq, which is serviced by a > > > kthread. As kthreads never return to user space, they usually don't have > > > an mm, and so we can use the existing infrastructure to swap in the > > > efi_mm while the EFI call is in progress. This is visible to the > > > scheduler, which will therefore reactivate the selected mm when > > > switching out the kthread and back in again. > > > > > > Given that the EFI spec explicitly permits runtime services to be called > > > with interrupts enabled, firmware code is already required to tolerate > > > interruptions. So rather than disable preemption, disable only migration > > > so that EFI runtime services are less likely to cause scheduling delays. > > > > > > Note, though, that the firmware executes at the same privilege level as > > > the kernel, and is therefore able to disable interrupts altogether. > > > > Is the migrate_disable() strictly required, or just paranoia? > > > > Runtime services might be polling the secure firmware for an async > completion when they are interrupted, and so I don't think it is > generally safe to assume that an interrupted EFI runtime service can > be resumed on another CPU. Can we please get a comment with that migrate_disable() explaining this? Thanks!