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 D04CD29B23E; Fri, 11 Jul 2025 13:48:39 +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=1752241721; cv=none; b=Z6Oz8uE+LPch8TD1JhUSO9+/8jTR4IZiCf2T+4W1iAu0+JaaHPQ+ZmDZkxF98MrpvbAICDQxSoIpQSkQlQ76WuvoiM4K5UmM9DmrBO685TWnKj0WyZfiOmbIj6pGw1OxbjG/mbcWay0NMGxBroWd6exy1QdiRIu/KnxabWBZLco= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1752241721; c=relaxed/simple; bh=IHqigtY29yY7gPTIcs+da8MtQgmREaPNajbBlRNBv+M=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=C/PacXjsprEk/+yPjhpMA8s5kCL5lEfVpqWBDJbcGzE/IBD1HQ4wQSHGJ/wNimJbGEBVHf71FARw8t7sgM2lWYBE8Xz7azt2TC6rIZB/exLd+l13xkCLJKH1Pf1H9w4kX0gmEaROK7Krlpg+FebMZX0HkhfzH/3Zkh3xHqw1TRE= 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=ILSyCxPb; 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="ILSyCxPb" 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=tUH0tIJOVwWDFNy2+9dmZOhPrE/5Fvoqxfp+0bTHgrI=; b=ILSyCxPbk9iapgeHlg4wA4zdDm lO8IwymKmEFClv4NLy7s1kVZAteH8c9eQg/Zw80qWFIqwNkSVOvx1Kcofm7oF5VOxM7OjiTMLyukj 6CrTpwLxe2sdVRcN1KJN39vynhDYc4PPOlttlR/7/iOcMcDts42+IvYh+bLP5mVA6gvyj47/vwQdV hqzmPYBzqd3LWdPotluHFfUBwtgaVNc+xuZ01DVi11lGFk7LCRkNsrWHtI16FKBXjf5Aqn+lPmQKL 07mbIPOyVpRo6s9rCf4R/I2+myctI6Os1ssVStTLFPhaXGo4DPV1QV/RzcanSRYTmPS7j71utJ33m uP3ltx1w==; 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 1uaE77-0000000DBNW-3Hvg; Fri, 11 Jul 2025 13:48:34 +0000 Received: by noisy.programming.kicks-ass.net (Postfix, from userid 1000) id 2C35D3001AA; Fri, 11 Jul 2025 15:48:33 +0200 (CEST) Date: Fri, 11 Jul 2025 15:48:33 +0200 From: Peter Zijlstra To: Ard Biesheuvel Cc: linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, Ard Biesheuvel , Will Deacon , Mark Rutland , Sebastian Andrzej Siewior Subject: Re: [RFC PATCH 7/7] arm64/efi: Call EFI runtime services without disabling preemption Message-ID: <20250711134833.GI905792@noisy.programming.kicks-ass.net> References: <20250514174339.1834871-9-ardb+git@google.com> <20250514174339.1834871-16-ardb+git@google.com> 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: <20250514174339.1834871-16-ardb+git@google.com> 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? > Signed-off-by: Ard Biesheuvel > --- > arch/arm64/kernel/efi.c | 15 +++++++++++++-- > 1 file changed, 13 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c > index 5d188c6c44d7..1c86a891f6d7 100644 > --- a/arch/arm64/kernel/efi.c > +++ b/arch/arm64/kernel/efi.c > @@ -10,6 +10,7 @@ > #include > #include > #include > +#include > #include > #include > > @@ -176,7 +177,12 @@ bool arch_efi_call_virt_setup(void) > if (WARN_ON(down_trylock(&efi_rt_lock))) > return false; > > - efi_virtmap_load(); > + if (preemptible() && (current->flags & PF_KTHREAD)) { > + migrate_disable(); > + kthread_use_mm(&efi_mm); > + } else { > + efi_virtmap_load(); > + } > uaccess_ttbr0_enable(); > post_ttbr_update_workaround(); > __efi_fpsimd_begin(); > @@ -186,7 +192,12 @@ bool arch_efi_call_virt_setup(void) > void arch_efi_call_virt_teardown(void) > { > __efi_fpsimd_end(); > - efi_virtmap_unload(); > + if (preemptible() && (current->flags & PF_KTHREAD)) { > + kthread_unuse_mm(&efi_mm); > + migrate_enable(); > + } else { > + efi_virtmap_unload(); > + } > uaccess_ttbr0_disable(); > up(&efi_rt_lock); > } > -- > 2.49.0.1101.gccaa498523-goog >