From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 31DEB4921A2 for ; Tue, 3 Mar 2026 15:45:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772552723; cv=none; b=Vg5Yg/VzSEDTSLHxIr65aRVajB9PjNfYTduW7ScKTcsTEWN+NuS1jDOvvKecvOkzJJdKllGoJODwbcx9rvzM+LYQHqFCoQQkVfJgIaPPQBkMCHAOExpFleADddDNLSSVtLpCgnT2XdZiVFGUOsLtQiIiN3iW+3buDxQLTD9PJMg= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1772552723; c=relaxed/simple; bh=+qYGDeAo5eegZCpqIhfrWXuC4f7UNhUIlijkWimNlEo=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=cO8wpsRl1HQwLDfBV1HEBCAzrnlrnd9O8Ggu3d+mqvsOSz1PpHZPmfeniKF3tlafbb0b1giVBPZ+cvugpYZ4SLHp5ORfLK4KcijLD50f18N27Ibi8BEkGSR7ijsUsup8rfpRTnz8cn7wGaIVKp4GV16Z4RI0Wboo0L0fuIFbJWk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 201F4339; Tue, 3 Mar 2026 07:45:14 -0800 (PST) Received: from arm.com (usa-sjc-mx-foss1.foss.arm.com [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 6EE8F3F73B; Tue, 3 Mar 2026 07:45:18 -0800 (PST) Date: Tue, 3 Mar 2026 15:45:15 +0000 From: Catalin Marinas To: Will Deacon Cc: Linus Walleij , Marc Zyngier , Oliver Upton , Joey Gouly , Suzuki K Poulose , Zenghui Yu , Ryan Roberts , Ankur Arora , David Hildenbrand , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, James Clark Subject: Re: [PATCH] arm64: Implement clear_pages() Message-ID: References: <20260303-aarch64-clear-pages-v1-1-ad0c3ee9a555@kernel.org> Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Tue, Mar 03, 2026 at 02:46:34PM +0000, Will Deacon wrote: > On Tue, Mar 03, 2026 at 11:06:13AM +0100, Linus Walleij wrote: > > On QEMU: > > > > Before this patch: After this patch: > > 2.38 GB/s 2.41 GB/s > > I really don't think we should pay attention to performance under QEMU > as it doesn't necessarily have any correlation with real hardware. I agree. > > diff --git a/arch/arm64/include/asm/page.h b/arch/arm64/include/asm/page.h > > index b39cc1127e1f..916a3e7c9a19 100644 > > --- a/arch/arm64/include/asm/page.h > > +++ b/arch/arm64/include/asm/page.h > > @@ -20,7 +20,18 @@ struct page; > > struct vm_area_struct; > > > > extern void copy_page(void *to, const void *from); > > -extern void clear_page(void *to); > > +extern void clear_pages_asm(void *addr, unsigned int nbytes); > > + > > +static inline void clear_pages(void *addr, unsigned int npages) > > +{ > > + clear_pages_asm(addr, npages * PAGE_SIZE); > > +} > > +#define clear_pages clear_pages > > Hmm. From what I can tell, this just turns a branch in C code into a > branch in assembly, so it's hard to correlate that meaningfully with > the performance improvement you see. > > If we have CPUs that are this sensitive to branches, perhaps we'd be > better off taking the opposite approach and moving more code into C > so that the compiler can optimise the control flow for us? I think it's more than the loop branch - the whole DCZID_EL0 read to decide whether to use DC ZVA or STNP. I wonder why we didn't do that with an alternative than always read the sysreg. That said, I wouldn't mind rewriting this in C if the numbers don't get worse. It is a bit more involved if we keep the DC ZVA use, though with alternatives maybe not that bad (mte_set_mem_tag_range() is an example of doing something similar in C but for clear page we don't need to deal with unaligned boundaries). -- Catalin