From mboxrd@z Thu Jan 1 00:00:00 1970 From: David Vrabel Subject: Re: [PATCHv9 3/4] gnttab: make the grant table lock a read-write lock Date: Fri, 22 May 2015 09:54:02 +0100 Message-ID: <555EEEAA.4040203@citrix.com> References: <1432137250-3700-1-git-send-email-david.vrabel@citrix.com> <1432137250-3700-4-git-send-email-david.vrabel@citrix.com> <555DD049020000780007CAD2@mail.emea.novell.com> <555DDF4E.7030508@citrix.com> <555E0D7A020000780007CCBF@mail.emea.novell.com> <555DF6BE.20102@citrix.com> <555EEADA020000780007D045@mail.emea.novell.com> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Received: from mail6.bemta14.messagelabs.com ([193.109.254.103]) by lists.xen.org with esmtp (Exim 4.72) (envelope-from ) id 1YviiZ-0005YQ-Ta for xen-devel@lists.xenproject.org; Fri, 22 May 2015 08:54:12 +0000 In-Reply-To: <555EEADA020000780007D045@mail.emea.novell.com> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xen.org Errors-To: xen-devel-bounces@lists.xen.org To: Jan Beulich , David Vrabel Cc: xen-devel@lists.xenproject.org, Keir Fraser , Ian Campbell , Tim Deegan List-Id: xen-devel@lists.xenproject.org On 22/05/15 07:37, Jan Beulich wrote: >>>> On 21.05.15 at 17:16, wrote: >> On 21/05/15 15:53, Jan Beulich wrote: >>>>>> On 21.05.15 at 15:36, wrote: >>>> On 21/05/15 11:32, Jan Beulich wrote: >>>>>>>> On 20.05.15 at 17:54, wrote: >>>>>> @@ -827,9 +828,11 @@ __gnttab_map_grant_ref( >>>>>> if ( (wrc + rdc) == 0 ) >>>>>> err = iommu_map_page(ld, frame, frame, IOMMUF_readable); >>>>>> } >>>>>> + >>>>>> + double_gt_lock(lgt, rgt); >>>>> >>>>> unlock. And with this code path actually used (due to the bug it's >>>>> pretty clear it didn't get exercised in your testing), how does >>>>> performance look like? >>>> >>>> I think it will be no worse than what it was before -- this path already >>>> really sucks (mapcount() loops over 1000s of entries). I don't care >>>> about this path at all. >>> >>> It's kind of strange that you don't care about this case - afaict we're >>> not getting there by default solely because dom0-strict mode is not >>> currently the default (albeit arguably it should be). >> >> What's your point? Are you going to refuse this series unless it also >> optimizes dom0-strict mode? > > No, I'm certainly not going to be. But this series of "I don't care" > worry me to a certain degree (apart from making me wonder > why, as so far I thought an as secure as possible environment is > key for XenServer, and "iommu=dom0-strict" is certainly helping > with that, which is also why I said that this should arguably > become the default). dom0-strict mode doesn't align with our PV-IOMMU plans to make BFN == PFN (including for grants). David