From mboxrd@z Thu Jan 1 00:00:00 1970 From: Paul Brook Subject: Re: [Qemu-devel] Re: [PATCH] virtio-blk: add SGI_IO passthru support Date: Thu, 30 Apr 2009 22:49:19 +0100 Message-ID: <200904302249.20940.paul@codesourcery.com> References: <20090427082606.GA32604@lst.de> <200904291237.21558.paul@codesourcery.com> <20090430201350.GA30619@lst.de> Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit Cc: qemu-devel@nongnu.org, Anthony Liguori , kvm@vger.kernel.org, Rusty Russell , Christian Borntraeger , Hannes Reinecke To: Christoph Hellwig Return-path: Received: from mail.codesourcery.com ([65.74.133.4]:32981 "EHLO mail.codesourcery.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754688AbZD3VtX (ORCPT ); Thu, 30 Apr 2009 17:49:23 -0400 In-Reply-To: <20090430201350.GA30619@lst.de> Content-Disposition: inline Sender: kvm-owner@vger.kernel.org List-ID: On Thursday 30 April 2009, Christoph Hellwig wrote: > On Wed, Apr 29, 2009 at 12:37:20PM +0100, Paul Brook wrote: > > How exactly does it introduce additional latency? A scsi command block is > > hardly large or complicated. Are you suggesting that a 16/32byte scsi > > command takes significantly longer to process than a 16byte virtio > > command descriptor? I'd expect any extra processing to be a small > > fraction of the host syscall latency, let alone the latency of the > > physical host adapter. It probably even fits on the same CPU cache line. > > Encoding the scsi CDB is additional work but I would be surprised it it > is mesurable. Just using scsi cdbs would be simple enough, the bigger > issue is emulating a full blown scsi bus because then you need to do all > kinds queueing decisions at target levels etc and drag in a complicated > scsi stack and not just a simple block driver in the guest. And at > least on current linux kernels that does introduce mesurable latency. Only if you emulate a crufty old parallel scsi bus, and that's just silly. One of the nice things about scsi is it separates the command set from the transport layer. cf. USB mass-storage, SAS, SBP2(firewire), and probably several others I've forgotten. Paul