From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:56000) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZEtr-00036e-6g for qemu-devel@nongnu.org; Fri, 26 Feb 2016 04:41:28 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1aZEtn-0006T5-6Y for qemu-devel@nongnu.org; Fri, 26 Feb 2016 04:41:27 -0500 Received: from mail-wm0-x234.google.com ([2a00:1450:400c:c09::234]:36952) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1aZEtm-0006Sz-VO for qemu-devel@nongnu.org; Fri, 26 Feb 2016 04:41:23 -0500 Received: by mail-wm0-x234.google.com with SMTP id g62so62507539wme.0 for ; Fri, 26 Feb 2016 01:41:22 -0800 (PST) Sender: Paolo Bonzini References: <874mcvhpt5.fsf@blackfin.pond.sub.org> From: Paolo Bonzini Message-ID: <56D01DBF.8040700@redhat.com> Date: Fri, 26 Feb 2016 10:41:19 +0100 MIME-Version: 1.0 In-Reply-To: <874mcvhpt5.fsf@blackfin.pond.sub.org> Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] Question on ioeventfd behavior List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Markus Armbruster Cc: qemu-devel@nongnu.org On 26/02/2016 08:51, Markus Armbruster wrote: > /** > * memory_region_add_eventfd: Request an eventfd to be triggered when a word > * is written to a location. > * > * Marks a word in an IO region (initialized with memory_region_init_io()) > * as a trigger for an eventfd event. The I/O callback will not be called. > * The caller must be prepared to handle failure (that is, take the required > * action if the callback _is_ called). > * > * @mr: the memory region being updated. > * @addr: the address within @mr that is to be monitored > * @size: the size of the access to trigger the eventfd > * @match_data: whether to match against @data, instead of just @addr > * @data: the data to match against the guest write > * @fd: the eventfd to be triggered when @addr, @size, and @data all match. > **/ > void memory_region_add_eventfd(MemoryRegion *mr, > hwaddr addr, > unsigned size, > bool match_data, > uint64_t data, > EventNotifier *e); > > What exactly does "The caller must be prepared to handle failure" mean? > What are the failure modes? I think it simply meant that the caller should be ready for the case where ioeventfd is _not_ implemented (e.g. when running on TCG) and the MMIO callback is invoked anyway. memory.c now implements a generic fallback in memory_region_dispatch_write_eventfds, so that should not be necessary anymore. I cannot think of other meanings of "failure mode". In any case, ivshmem_io_write helpfully does the required event_notifier_set, so you're good. > Does "value written doesn't match" count as failure in the sense of this > clause? Rephrasing my question: what happens when the guest writes a > value to the Doorbell register that does not match any of the values > registered with memory_region_add_eventfd()? Is the I/O callback > called exactly as if ioeventfd was not in use? Yes, the latter. Paolo