From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=170.10.129.124 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794474940; b=gJF3VTuVj/WnOYpD7A7q5IopVUhGKMISdloZ0mPRv1ZwLmVhQwaMrrdJQr3X6W8VgScs 3p/HGryFLgv36cd4MXvhffomNhbLmADYF1yKFJUB5Ajwn/+hWbxtx0smfhw+5ZZucnWCO hR98YmjuP++e3EEv3lS2+R+QojkGILXirKGz6WwOpbsIkwesuqlh3/CmpSZ4BxW5D8S7u YSknDMTFOsrpvk6COfJP/PZlLnf9hFtMvSwz/t4fHO4PntDbWfOwQLx3OtL4n7egKk2fK Pch2ICbpwgLUjh7tNqc1zws3RveG7tQJtZ/S7FBTaW4roKQcnQfRh4XQkjI8o+Ujo8w== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794474940; h=DMARC-Filter:DKIM-Signature:Received:X-MC-Unique: X-Mimecast-MFC-AGG-ID:Received:DKIM-Signature:X-Google-DKIM-Signature: X-Forwarded-Encrypted:X-Gm-Message-State:X-Gm-Gg:X-Received: X-Received:Received:Date:From:To:Cc:Subject:Message-ID:References: MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=in0wT3kSv8v4L0CNiG/ZIgaYFBbYCntKXbAs/Vd5cP4=; b=4EZBFpgS5kUgsaai+P7846qKrH7u6GhjXbLl86s8tOHpgIwZyLqTn0tIK8FkEy+1e5z9 RgP07FMsp29kVopNIkPhQOnZjxLlM2aJf2ZTMC/JGrPEhKmEMcPnUEDZOdoqPl4IZ+bX8 cJBb+qJVbuT9kpVM/Xu90FTYml2EGjiRb1CF/iP5yQTMdpd/WSMY5sBMU2JQv4nwKlJoD +XZZr4nr05u7F51CmfHpQmtTXo66C7m0XnXTY8OH2AmQzUybp2I+u3ifkq7qQwqg94I3I sKkdpRKxtARIh0VZ4sY8LCkFUi9L8ksx1qUDARx2IsNYFL4WyHIM95kJoytp1TZjoYg== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=aMuTvq3q; dkim=pass header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=google header.b=RBjixnsW; arc=none smtp.remote-ip=170.10.129.124 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 82590606A8AC for ; Thu, 12 Nov 2026 09:15:39 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 82590606A8AC Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=aMuTvq3q; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=google header.b=RBjixnsW DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1794474938; 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: in-reply-to:in-reply-to:references:references; bh=in0wT3kSv8v4L0CNiG/ZIgaYFBbYCntKXbAs/Vd5cP4=; b=aMuTvq3qIoH43sv+1a7P8mAUyk7hzIxGncQhFZS7l5317fpGTaPqEkVvZk8AEWtDZgKSf/ TaUUy3y1dNhIVRxz2oO0WwtmwY2WePLpuTTo+D0BoeK6BYN8YQ/YcUyZS/yDPgRp54OsGX Tk62qEa3stwnAzxKVZq2EX1rI2opIuU= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-113-4LYXOo65Ohqw3GmhrzZDKw-1; Thu, 12 Nov 2026 04:15:37 -0500 X-MC-Unique: 4LYXOo65Ohqw3GmhrzZDKw-1 X-Mimecast-MFC-AGG-ID: 4LYXOo65Ohqw3GmhrzZDKw_1794474936 Received: by mail-wr1-f69.google.com with SMTP id ffacd0b85a97d-4916d47eeb5so3279844f8f.1 for ; Thu, 12 Nov 2026 01:15:36 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1794474936; x=1795079736; darn=vger.kernel.org; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:from:to:cc:subject :date:message-id:reply-to:content-type; bh=in0wT3kSv8v4L0CNiG/ZIgaYFBbYCntKXbAs/Vd5cP4=; b=RBjixnsW08WSzYaCIeMMgfDMxPNjBHIl29W/hMUL87tEOBmVisUX3PMetALgCVp1OX TZbZ1VfJRXWZ8BFWFchwd7tgIJoXRAMZ1nRX5repU5Q31o9xRq0O705WrzyZCRzxWddb YzmuGdP+BRePNQ1n67DeI4eA92tJVnyS+DaC8eQoIrM/soc8PR2bRDqAqpO4lWc9OeBY c5+uNuKmNSGznIGKIs8ZALZpia3CJOVSRzKC0zetFO/gA8PKmjmviNBxx4Fe7zOqIGOM D98ld0NhCtGmG7X8A60u7hB2G3VvOiQYhEqVBCgco8EMqeWC5jtGqCnPUtPrqYNvdkS7 AcfQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20260707; t=1794474936; x=1795079736; h=in-reply-to:content-disposition:content-type:mime-version :references:message-id:subject:cc:to:from:date:x-gm-gg :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to :content-type; bh=in0wT3kSv8v4L0CNiG/ZIgaYFBbYCntKXbAs/Vd5cP4=; b=fJLaeixNudM6+AoQ+fGFXgf5sxdoBrm8o7Ari97TVSSoRtX1y7i1NCAxMvWyD8m5Hx ClI360srpwIPwgEgyhc6hoCMGZX7L2/I4B47eLedEMXowJqJXS0fZO/d+HU752oZyTrr NMcqFleEp4KQvsjnVAnN2jcBsIXnQMUY09R7MKeNFqQbffkYOQTy839qfJO6AtnoxHaR swTa4J6JbCh19GNGlIUPqQ1JJnp0S1GSarcScqLjbqASk5INLWKBQQFzcs/TMlGZYB1g 8C5nNex9FBZbdoK2ldqZwO3MU5cbbTE3SU3SKVYEIFTfOu29FDKGGFSZutD7DvszxSRI EvuA== X-Forwarded-Encrypted: i=1; AL+hErHZ7HAb3E8mbjYGeowNs+Rg/nhmnvR9XBeWzvoOQQPu6OqUNjoLXoZyN3alydaetMavTKc=@vger.kernel.org X-Gm-Message-State: APuU4Z8RRGX8pZXA2xpsHY1lQ8IF3OnzkyxMG7eL7rDW7go3Da4oAJwP /PRh4NAiArPX+gAMrN+oxflAHWPjyy2LtQyPDjVPutdNpTB17d6cRbzgWmDKzxIcnUPdoNpBV0x EWcNjZqQvYzwFxNcBBOPLmh35NTnF0QSwj32fj8/j7a7/cFngSCi8aw== X-Gm-Gg: AYBFou1HoSlW33eSOl1U1avptiMIjDkEWTUjXs0C7GrNtBj9s8nkauys3+uxNS16hpO lRQvXsZFE+Ma1gTJAhlLPpfYcVthjH7Zuzc8v7Wu3ETMOHRwYWW0AwGuYBXeDamtYBxItzpIfPz pOlovUOl/qH3oKNQtNzDuu6DyXTHnKcxJ6G2uapePAIfjq3nTHfHtMoqGJqII3PoPmugEcj3XB3 qdxShAPexTh7ZfWKnPqy/qdGKGkFi23vK0CMNNHjmAcaPna7HSXHkOkOI255dlifXI1Mkvbvpdu hgq8LFFHy0cukmp6zhHaMC34qWFBvvEiMPz+2EcCs5NYg3j84+HZdDXIZIUQZok++DOiBeww0ol 9J3neLCdVbcwzQMJtDmUaBD+mZkvUn30uTjn+Y2cprw== X-Received: by 2002:a05:6000:4b13:b0:487:88b:4681 with SMTP id ffacd0b85a97d-499c6027597mr7970526f8f.10.1794474935556; Thu, 12 Nov 2026 01:15:35 -0800 (PST) X-Received: by 2002:a05:6000:4b13:b0:487:88b:4681 with SMTP id ffacd0b85a97d-499c6027597mr7970448f8f.10.1794474934931; Thu, 12 Nov 2026 01:15:34 -0800 (PST) Received: from redhat.com ([2a0d:6fc0:1511:2900:ff37:b35c:a97:952d]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-498361f674fsm15808324f8f.33.2026.11.12.01.15.33 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 Nov 2026 01:15:34 -0800 (PST) Date: Thu, 12 Nov 2026 04:15:31 -0500 From: "Michael S. Tsirkin" To: zhaoguohan@kylinos.cn Cc: jasowangio@gmail.com, eperezma@redhat.com, xuanzhuo@linux.alibaba.com, kvm@vger.kernel.org, virtualization@lists.linux.dev, netdev@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH 1/2] vhost: set dirty log bits without pinning pages on x86 Message-ID: <20261112040926-mutt-send-email-mst@kernel.org> References: <20261112085631.128819-1-zhaoguohan@kylinos.cn> <20261112085631.128819-2-zhaoguohan@kylinos.cn> X-Mailing-List: kvm@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261112085631.128819-2-zhaoguohan@kylinos.cn> On Thu, Nov 12, 2026 at 04:56:30PM +0800, zhaoguohan@kylinos.cn wrote: > From: Khan Zhao > > set_bit_to_user() pins the log page with pin_user_pages_fast(), maps it > with kmap_atomic(), sets one bit and then unpins the page and marks it > dirty. This runs for every bit set in the dirty log, e.g. on each used > ring update while logging is enabled for live migration. > > log_write() always passes a byte address and a bit number in [0, 7], so > the update can be done as a byte-sized cmpxchg on the user address. On > x86, try that first using unsafe_try_cmpxchg_user() inside a > user_access_begin()/user_access_end() section with page faults disabled. > If the access faults (page not present, read-only or needing COW), or > the cmpxchg loses to concurrent updates of the same byte four times in > a row, fall back to the existing pin/kmap/set_bit path, which can fault > the page in. > > unsafe_try_cmpxchg_user() is only implemented on x86, so other > architectures keep using the existing path. > > On an x86_64 KVM guest (i7-12800HX host), a synthetic used-ring > workload with logging enabled went from 243.60 to 171.11 ns per > descriptor, averaged over five alternating runs. This is the > end-to-end cost per descriptor, not live migration performance. > > Signed-off-by: Khan Zhao > --- > drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++++----- > 1 file changed, 45 insertions(+), 5 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index e29f154706da..9c3045504709 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2527,11 +2527,7 @@ long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *argp) > } > EXPORT_SYMBOL_GPL(vhost_dev_ioctl); > > -/* TODO: This is really inefficient. We need something like get_user() > - * (instruction directly accesses the data, with an exception table entry > - * returning -EFAULT). See Documentation/arch/x86/exception-tables.rst. > - */ > -static int set_bit_to_user(int nr, void __user *addr) > +static int set_bit_to_user_slow(int nr, void __user *addr) > { > unsigned long log = (unsigned long)addr; > struct page *page; > @@ -2550,6 +2546,50 @@ static int set_bit_to_user(int nr, void __user *addr) > return 0; > } > > +#ifdef CONFIG_X86 > +static bool set_bit_to_user_fast(int nr, void __user *addr) > +{ > + u8 __user *p = addr; > + u8 old; > + int retries = 4; > + bool success = false; > + > + /* > + * log_write() passes a byte address and a bit number in [0, 7]. > + * On x86, update just that byte to avoid crossing a page boundary. > + * Bound retries so a contended byte cannot hold up the worker. > + */ > + pagefault_disable(); > + if (!user_access_begin(p, sizeof(*p))) > + goto out; > + unsafe_get_user(old, p, fault); > + do { > + if (unsafe_try_cmpxchg_user(p, &old, old | BIT(nr), fault)) { > + success = true; > + break; > + } > + } while (--retries); > +fault: > + user_access_end(); > +out: > + pagefault_enable(); > + return success; > +} Yes we wanted this for the longest time! But can we use the nice scoped_user_rw_access API? Something like: pagefault_disable(); scoped_user_rw_access(p, efault) { unsafe_get_user(old, p, efault); do { if (unsafe_try_cmpxchg_user(p, &old, old | BIT(nr), efault)) { success = true; break; } } while (--retries); } pagefault_enable(); return success; efault: pagefault_enable(); return false; maybe? > +#else > +static bool set_bit_to_user_fast(int nr, void __user *addr) > +{ > + return false; > +} > +#endif /* CONFIG_X86 */ > + > +static int set_bit_to_user(int nr, void __user *addr) > +{ > + if (set_bit_to_user_fast(nr, addr)) > + return 0; > + /* Resolve faults, or use the native bit layout on other architectures. */ > + return set_bit_to_user_slow(nr, addr); > +} > + > static int log_write(void __user *log_base, > u64 write_address, u64 write_length) > { > -- > 2.43.0