From owner-svn-src-user@FreeBSD.ORG Mon Dec 31 18:28:39 2012 Return-Path: Delivered-To: svn-src-user@freebsd.org Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 0FE7EB8E; Mon, 31 Dec 2012 18:28:39 +0000 (UTC) (envelope-from mr.kodiak@gmail.com) Received: from mail-ie0-f176.google.com (mail-ie0-f176.google.com [209.85.223.176]) by mx1.freebsd.org (Postfix) with ESMTP id A25C78FC13; Mon, 31 Dec 2012 18:28:38 +0000 (UTC) Received: by mail-ie0-f176.google.com with SMTP id 13so15226494iea.35 for ; Mon, 31 Dec 2012 10:28:32 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20120113; h=mime-version:sender:in-reply-to:references:from:date :x-google-sender-auth:message-id:subject:to:cc:content-type; bh=XGzvIG7UnTmJNMbV9LKosKN+rG1H61VucE6P0x+V2lw=; b=eryyAdtapp7oOv1uoF7LeGUY6OtRRyhDow2qgXTvCG94VU7dm/nVE672C2JWHV9Vxs mICdrviVrN/Cy4ECjO6SYOtblDwEpPuvg2zYy5W2RTzEBO1x+xXTv89OSj/a9NyfzopP kGJy0lWPXuwtm18qBgJW6ZCMFHeYLOt9zfzs9ubebLtoJG/uN7wYbu7NuomJikUObFRB /UHmbgddvKCIi7DVU451IMeSqU+utg25n5Dpw4xKHKlQauQKpi3WqRRoB3QAqSUe/3vB uICVZB9ioG67ZL2fVo34gG1vk8I4G0wasOgRVtSRGTGEf1MvwShanlLaKxePx4fFGTsp KXtg== Received: by 10.50.219.169 with SMTP id pp9mr35395333igc.46.1356978512697; Mon, 31 Dec 2012 10:28:32 -0800 (PST) MIME-Version: 1.0 Sender: mr.kodiak@gmail.com Received: by 10.64.142.198 with HTTP; Mon, 31 Dec 2012 10:28:02 -0800 (PST) In-Reply-To: References: <201212282218.qBSMIfX2015054@svn.freebsd.org> <20121228223259.GX82219@kib.kiev.ua> <20121228231649.GZ82219@kib.kiev.ua> <20121229150107.GC82219@kib.kiev.ua> <20121229160355.GD82219@kib.kiev.ua> From: Bryan Venteicher Date: Mon, 31 Dec 2012 12:28:02 -0600 X-Google-Sender-Auth: lKoz1Gt_VKeRi9DwQNa14QDBNGg Message-ID: Subject: Re: svn commit: r244793 - in user/attilio/membarclean/dev: drm drm2 netmap virtio To: attilio@freebsd.org Content-Type: text/plain; charset=ISO-8859-1 Cc: src-committers@freebsd.org, svn-src-user@freebsd.org X-BeenThere: svn-src-user@freebsd.org X-Mailman-Version: 2.1.14 Precedence: list List-Id: "SVN commit messages for the experimental " user" src tree" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 31 Dec 2012 18:28:39 -0000 On Sun, Dec 30, 2012 at 7:33 PM, Attilio Rao wrote: > On Sun, Dec 30, 2012 at 9:00 PM, Bryan Venteicher wrote: >> On Sat, Dec 29, 2012 at 10:37 AM, Attilio Rao wrote: >>> On Sat, Dec 29, 2012 at 5:03 PM, Konstantin Belousov >>> wrote: >>>> On Sat, Dec 29, 2012 at 07:13:57AM -0800, Attilio Rao wrote: >>>>> On Sat, Dec 29, 2012 at 7:01 AM, Konstantin Belousov >>>>> wrote: >>>>> > On Fri, Dec 28, 2012 at 11:34:39PM +0000, Attilio Rao wrote: >>>>> >> On Fri, Dec 28, 2012 at 11:16 PM, Konstantin Belousov >>>>> >> wrote: >>>>> >> > On Fri, Dec 28, 2012 at 10:55:47PM +0000, Attilio Rao wrote: >>>>> >> >> On Fri, Dec 28, 2012 at 10:32 PM, Konstantin Belousov >>>>> >> >> wrote: >>>>> >> >> > On Fri, Dec 28, 2012 at 10:18:41PM +0000, Attilio Rao wrote: >>>>> >> >> >> Author: attilio >>>>> >> >> >> Date: Fri Dec 28 22:18:41 2012 >>>>> >> >> >> New Revision: 244793 >>>>> >> >> >> URL: http://svnweb.freebsd.org/changeset/base/244793 >>>>> >> >> >> >>>>> >> >> >> Log: >>>>> >> >> >> - Remove rmb() usage from netmap and replace it with intended operation >>>>> >> >> >> to do actual memory fetching reads. >>>>> >> >> >> - GC unused DRM_WRITEMEMORYBARRIER() from drm and drm2. >>>>> >> >> >> - Use atomic_load_acq_*() in virtio and drm2 in places that don't need >>>>> >> >> >> to use rmb(). >>>>> >> >> >> >>>>> >> >> >> All these changes remove completely rmb() from MI code, with the >>>>> >> >> >> exception of cxgbe which will be hammered in a followup commit. >>>>> >> >> >> >>>>> >> >> >> Modified: >>>>> >> >> >> user/attilio/membarclean/dev/drm/drmP.h >>>>> >> >> >> user/attilio/membarclean/dev/drm2/drmP.h >>>>> >> >> >> user/attilio/membarclean/dev/drm2/drm_atomic.h >>>>> >> >> >> user/attilio/membarclean/dev/drm2/drm_irq.c >>>>> >> >> >> user/attilio/membarclean/dev/netmap/netmap.c >>>>> >> >> >> user/attilio/membarclean/dev/virtio/virtqueue.c >>>>> >> >> >> >>>>> >> >> >> Modified: user/attilio/membarclean/dev/drm/drmP.h >>>>> >> >> >> ============================================================================== >>>>> >> >> >> --- user/attilio/membarclean/dev/drm/drmP.h Fri Dec 28 22:06:50 2012 (r244792) >>>>> >> >> >> +++ user/attilio/membarclean/dev/drm/drmP.h Fri Dec 28 22:18:41 2012 (r244793) >>>>> >> >> >> @@ -241,11 +241,9 @@ typedef u_int32_t u32; >>>>> >> >> >> typedef u_int16_t u16; >>>>> >> >> >> typedef u_int8_t u8; >>>>> >> >> >> >>>>> >> >> >> -/* DRM_READMEMORYBARRIER() prevents reordering of reads. >>>>> >> >> >> - * DRM_WRITEMEMORYBARRIER() prevents reordering of writes. >>>>> >> >> >> +/* DRM_WRITEMEMORYBARRIER() prevents reordering of writes. >>>>> >> >> >> * DRM_MEMORYBARRIER() prevents reordering of reads and writes. >>>>> >> >> >> */ >>>>> >> >> >> -#define DRM_READMEMORYBARRIER() rmb() >>>>> >> >> >> #define DRM_WRITEMEMORYBARRIER() wmb() >>>>> >> >> >> #define DRM_MEMORYBARRIER() mb() >>>>> >> >> >> >>>>> >> >> >> >>>>> >> >> >> Modified: user/attilio/membarclean/dev/drm2/drmP.h >>>>> >> >> >> ============================================================================== >>>>> >> >> >> --- user/attilio/membarclean/dev/drm2/drmP.h Fri Dec 28 22:06:50 2012 (r244792) >>>>> >> >> >> +++ user/attilio/membarclean/dev/drm2/drmP.h Fri Dec 28 22:18:41 2012 (r244793) >>>>> >> >> >> @@ -263,11 +263,9 @@ typedef int32_t s32; >>>>> >> >> >> typedef int16_t s16; >>>>> >> >> >> typedef int8_t s8; >>>>> >> >> >> >>>>> >> >> >> -/* DRM_READMEMORYBARRIER() prevents reordering of reads. >>>>> >> >> >> - * DRM_WRITEMEMORYBARRIER() prevents reordering of writes. >>>>> >> >> >> +/* DRM_WRITEMEMORYBARRIER() prevents reordering of writes. >>>>> >> >> >> * DRM_MEMORYBARRIER() prevents reordering of reads and writes. >>>>> >> >> >> */ >>>>> >> >> >> -#define DRM_READMEMORYBARRIER() rmb() >>>>> >> >> >> #define DRM_WRITEMEMORYBARRIER() wmb() >>>>> >> >> >> #define DRM_MEMORYBARRIER() mb() >>>>> >> >> >> >>>>> >> >> >> >>>>> >> >> >> Modified: user/attilio/membarclean/dev/drm2/drm_atomic.h >>>>> >> >> >> ============================================================================== >>>>> >> >> >> --- user/attilio/membarclean/dev/drm2/drm_atomic.h Fri Dec 28 22:06:50 2012 (r244792) >>>>> >> >> >> +++ user/attilio/membarclean/dev/drm2/drm_atomic.h Fri Dec 28 22:18:41 2012 (r244793) >>>>> >> >> >> @@ -38,6 +38,7 @@ typedef u_int32_t atomic_t; >>>>> >> >> >> >>>>> >> >> >> #define atomic_set(p, v) (*(p) = (v)) >>>>> >> >> >> #define atomic_read(p) (*(p)) >>>>> >> >> >> +#define atomic_read_acq(p) atomic_load_acq_int(p) >>>>> >> >> >> #define atomic_inc(p) atomic_add_int(p, 1) >>>>> >> >> >> #define atomic_dec(p) atomic_subtract_int(p, 1) >>>>> >> >> >> #define atomic_add(n, p) atomic_add_int(p, n) >>>>> >> >> >> >>>>> >> >> >> Modified: user/attilio/membarclean/dev/drm2/drm_irq.c >>>>> >> >> >> ============================================================================== >>>>> >> >> >> --- user/attilio/membarclean/dev/drm2/drm_irq.c Fri Dec 28 22:06:50 2012 (r244792) >>>>> >> >> >> +++ user/attilio/membarclean/dev/drm2/drm_irq.c Fri Dec 28 22:18:41 2012 (r244793) >>>>> >> >> >> @@ -701,8 +701,7 @@ u32 drm_vblank_count_and_time(struct drm >>>>> >> >> >> do { >>>>> >> >> >> cur_vblank = atomic_read(&dev->_vblank_count[crtc]); >>>>> >> >> >> *vblanktime = vblanktimestamp(dev, crtc, cur_vblank); >>>>> >> >> >> - rmb(); >>>>> >> >> >> - } while (cur_vblank != atomic_read(&dev->_vblank_count[crtc])); >>>>> >> >> >> + } while (cur_vblank != atomic_read_acq(&dev->_vblank_count[crtc])); >>>>> >> >> >> >>>>> >> >> >> return cur_vblank; >>>>> >> >> >> } >>>>> >> >> > >>>>> >> >> > The drm/drm2 is the contributed code, maintained outside the tree, and I >>>>> >> >> > explicitely decided to keep the original approach. I will appreciate if >>>>> >> >> > this would be left as it is. >>>>> >> >> >>>>> >> >> Aren't the stubs in atomic FreeBSD specific? >>>>> >> >> So it does rely on some sort of Linux layer emulation? >>>>> >> >> Right now my target is to get rid of rmb() and wmb() and garbage >>>>> >> >> collect them. If drm2 is Linux specific and needs to stay that way I >>>>> >> >> will make a different patch. >>>>> >> > >>>>> >> > *mb() are the Linux KPI, and almost any non-trivial ported driver >>>>> >> > depends on them. The mess which we already have in tree with the >>>>> >> > duplicated and sometimes wrong definitions of the barries is because we >>>>> >> > get this compat functions too late, and the driver authors added them >>>>> >> > locally. >>>>> >> > >>>>> >> > The functions should stay there, and their usage in the ported drivers too. >>>>> >> > Otherwise, the mess will reappear and continue to spread. >>>>> >> >>>>> >> I don't agree with "the functions should stay there" just for your >>>>> >> premise: they are not a FreeBSD KPI and they should be defined in >>>>> >> atomic.h. They violate our atomic/membar model and they are only >>>>> >> useful for Linux-feature emulation purposes. More importantly, they >>>>> >> are mostly wrong and the requirements are niether clear or precise >>>>> >> because they makes just no sense in FreeBSD. >>>>> > In what way they are wrong ? >>>>> > >>>>> > I am fine with removing them from the machine/atomic.h and putting >>>>> > under any other namespace, even into machine/never_use_the_linuxisms.h. >>>>> > But driver authors must not be forced to reimplement the same KPI >>>>> > Nth time. >>>>> >>>>> This is fine. >>>>> They are wrong because Linux requirements about write memory barriers >>>>> are different than our _rel barriers and their requirements about read >>>>> memory barriers are different than our _acq barriers. >>>>> Per-Linux doc, infact, their write barriers only serialize over other >>>>> pre-stores while their read barriers only serialize over post-loads. >>>>> FreeBSD serializes over pre-stores/loads (for _rel) and >>>>> post-stores/loads (for _acq). >>>> Yes, but this does not make Linux-style rmb/wmb barriers incompatible with >>>> the FreeBSD acq/rel barriers. They pairs have obviously different >>>> semantic, they are not interchangable, but there is nothing new to state. >>>> I do not see how the rmb/wmb can be claimed incompatible or wrong. >>>> They are only different. >>> >>> As first thing they are undocumented in our world. Second thing they >>> are different by freebsd philosophy. And I hardly believe all drivers >>> developer understands this nit differences between freebsd _rel/_acq >>> and wmb()/acq(), leaving alone architecture porters. >>> >>>>> >>>>> Having the wmb() and rmb() in linux compat stub, that all the drivers >>>>> can reuse, is a perfectly acceptable compromise. >>>>> However such layer doesn't exist nowadays. I think there were some >>>>> resistence in the past about implementing it, because people were >>>>> freightened that drivers weren't going to be ported natively to >>>>> FreeBSD anymore. However for simple stuff like membars this should not >>>>> withstand. >>>> Ok. >>>> >>>>> >>>>> > BTW, did you looked at the lib/libc/sys/__vdso_gettimeofday.c:binuptime() ? >>>>> > >>>>> >> >>>>> >> My current plan is to just keep mb() at the atomic-FreeBSD level and >>>>> >> get rid of the others. >>>> Still, there are situations where FreeBSD implementation does not work, >>>> see above example from libc. >>> >>> I still need to look into it, but there are also other cases, like the >>> virtio ones, where we want to get _rel or _acq semantic for smaller >>> than int read and writes. I think I will make a followup soon on all >>> the cases where the wmb() and rmb() are used, try to see where our >>> initial model is failing, if any, and try to formalize the situation >>> some more. This is a first attempt. >>> But supporting wmb() and rmb() at the architecture level only because >>> "Linux does it" is wrong. If they belong to a compat layer, it is >>> acceptable. >>> >> >> For VirtIO, I think I would be fine with reverting r240427 to make >> everything just straight mb()'s, and then #ifdef'ing atomic(9) for >> those arch's with 16-bit support. > > Can you take care of doing so? I don't plan any immediate action on > this item in the next weeks so any help will be appreciated. > At least add some comments saying while the wmb(), rmb() are used > would be helpful in understanding what are the constraints in general. > I'm currently swamped with work, but should be able take care of it in the next few weeks. > Attilio > > > -- > Peace can only be achieved by understanding - A. Einstein