From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751344AbeECHZm (ORCPT ); Thu, 3 May 2018 03:25:42 -0400 Received: from mx3-rdu2.redhat.com ([66.187.233.73]:59652 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751240AbeECHZj (ORCPT ); Thu, 3 May 2018 03:25:39 -0400 Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring To: Tiwei Bie , "Michael S. Tsirkin" Cc: virtualization@lists.linux-foundation.org, linux-kernel@vger.kernel.org, netdev@vger.kernel.org, wexu@redhat.com, jfreimann@redhat.com References: <20180425051550.24342-1-tiwei.bie@intel.com> <20180425051550.24342-5-tiwei.bie@intel.com> <34781052-df9f-e505-cd3f-08e460b34dcc@redhat.com> <20180502072819.mf5l3dypk6dwx2s7@debian> <20180502164828-mutt-send-email-mst@kernel.org> <20180502151255.h3x6rhszxa3euinl@debian> <20180502184015-mutt-send-email-mst@kernel.org> <20180503011116.qvoyblcpklinrk26@debian> <20180503044218-mutt-send-email-mst@kernel.org> <20180503020949.5u3qz32gsk33z6vk@debian> From: Jason Wang Message-ID: <9f0b4e37-63ff-42f9-f2e6-3747a19a0206@redhat.com> Date: Thu, 3 May 2018 15:25:29 +0800 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180503020949.5u3qz32gsk33z6vk@debian> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 2018年05月03日 10:09, Tiwei Bie wrote: >>>> So how about we use the straightforward way then? >>> You mean we do new += vq->vring_packed.num instead >>> of event_idx -= vq->vring_packed.num before calling >>> vring_need_event()? >>> >>> The problem is that, the second param (new_idx) of >>> vring_need_event() will be used for: >>> >>> (__u16)(new_idx - event_idx - 1) >>> (__u16)(new_idx - old) >>> >>> So if we change new, we will need to change old too. >> I think that since we have a branch there anyway, >> we are better off just special-casing if (wrap_counter != vq->wrap_counter). >> Treat is differenty and avoid casts. >> >>> And that would be an ugly hack.. >>> >>> Best regards, >>> Tiwei Bie >> I consider casts and huge numbers with two's complement >> games even uglier. > The dependency on two's complement game is introduced > since the split ring. > > In packed ring, old is calculated via: > > old = vq->next_avail_idx - vq->num_added; > > In split ring, old is calculated via: > > old = vq->avail_idx_shadow - vq->num_added; > > In both cases, when vq->num_added is bigger, old will > be a big number. > > Best regards, > Tiwei Bie > How about just do something like vhost: static u16 vhost_idx_diff(struct vhost_virtqueue *vq, u16 old, u16 new) {     if (new > old)         return new - old;     return  (new + vq->num - old); } static bool vhost_vring_packed_need_event(struct vhost_virtqueue *vq,                       __u16 event_off, __u16 new,                       __u16 old) {     return (__u16)(vhost_idx_diff(vq, new, event_off) - 1) <            (__u16)vhost_idx_diff(vq, new, old); } ? From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jason Wang Subject: Re: [RFC v3 4/5] virtio_ring: add event idx support in packed ring Date: Thu, 3 May 2018 15:25:29 +0800 Message-ID: <9f0b4e37-63ff-42f9-f2e6-3747a19a0206@redhat.com> References: <20180425051550.24342-1-tiwei.bie@intel.com> <20180425051550.24342-5-tiwei.bie@intel.com> <34781052-df9f-e505-cd3f-08e460b34dcc@redhat.com> <20180502072819.mf5l3dypk6dwx2s7@debian> <20180502164828-mutt-send-email-mst@kernel.org> <20180502151255.h3x6rhszxa3euinl@debian> <20180502184015-mutt-send-email-mst@kernel.org> <20180503011116.qvoyblcpklinrk26@debian> <20180503044218-mutt-send-email-mst@kernel.org> <20180503020949.5u3qz32gsk33z6vk@debian> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; Format="flowed" Content-Transfer-Encoding: base64 Cc: netdev@vger.kernel.org, wexu@redhat.com, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org To: Tiwei Bie , "Michael S. Tsirkin" Return-path: In-Reply-To: <20180503020949.5u3qz32gsk33z6vk@debian> Content-Language: en-US List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org List-Id: netdev.vger.kernel.org CgpPbiAyMDE45bm0MDXmnIgwM+aXpSAxMDowOSwgVGl3ZWkgQmllIHdyb3RlOgo+Pj4+IFNvIGhv dyBhYm91dCB3ZSB1c2UgdGhlIHN0cmFpZ2h0Zm9yd2FyZCB3YXkgdGhlbj8KPj4+IFlvdSBtZWFu IHdlIGRvIG5ldyArPSB2cS0+dnJpbmdfcGFja2VkLm51bSBpbnN0ZWFkCj4+PiBvZiBldmVudF9p ZHggLT0gdnEtPnZyaW5nX3BhY2tlZC5udW0gYmVmb3JlIGNhbGxpbmcKPj4+IHZyaW5nX25lZWRf ZXZlbnQoKT8KPj4+Cj4+PiBUaGUgcHJvYmxlbSBpcyB0aGF0LCB0aGUgc2Vjb25kIHBhcmFtIChu ZXdfaWR4KSBvZgo+Pj4gdnJpbmdfbmVlZF9ldmVudCgpIHdpbGwgYmUgdXNlZCBmb3I6Cj4+Pgo+ Pj4gKF9fdTE2KShuZXdfaWR4IC0gZXZlbnRfaWR4IC0gMSkKPj4+IChfX3UxNikobmV3X2lkeCAt IG9sZCkKPj4+Cj4+PiBTbyBpZiB3ZSBjaGFuZ2UgbmV3LCB3ZSB3aWxsIG5lZWQgdG8gY2hhbmdl IG9sZCB0b28uCj4+IEkgdGhpbmsgdGhhdCBzaW5jZSB3ZSBoYXZlIGEgYnJhbmNoIHRoZXJlIGFu eXdheSwKPj4gd2UgYXJlIGJldHRlciBvZmYganVzdCBzcGVjaWFsLWNhc2luZyBpZiAod3JhcF9j b3VudGVyICE9IHZxLT53cmFwX2NvdW50ZXIpLgo+PiBUcmVhdCBpcyBkaWZmZXJlbnR5IGFuZCBh dm9pZCBjYXN0cy4KPj4KPj4+IEFuZCB0aGF0IHdvdWxkIGJlIGFuIHVnbHkgaGFjay4uCj4+Pgo+ Pj4gQmVzdCByZWdhcmRzLAo+Pj4gVGl3ZWkgQmllCj4+IEkgY29uc2lkZXIgY2FzdHMgYW5kIGh1 Z2UgbnVtYmVycyB3aXRoIHR3bydzIGNvbXBsZW1lbnQKPj4gZ2FtZXMgZXZlbiB1Z2xpZXIuCj4g VGhlIGRlcGVuZGVuY3kgb24gdHdvJ3MgY29tcGxlbWVudCBnYW1lIGlzIGludHJvZHVjZWQKPiBz aW5jZSB0aGUgc3BsaXQgcmluZy4KPgo+IEluIHBhY2tlZCByaW5nLCBvbGQgaXMgY2FsY3VsYXRl ZCB2aWE6Cj4KPiBvbGQgPSB2cS0+bmV4dF9hdmFpbF9pZHggLSB2cS0+bnVtX2FkZGVkOwo+Cj4g SW4gc3BsaXQgcmluZywgb2xkIGlzIGNhbGN1bGF0ZWQgdmlhOgo+Cj4gb2xkID0gdnEtPmF2YWls X2lkeF9zaGFkb3cgLSB2cS0+bnVtX2FkZGVkOwo+Cj4gSW4gYm90aCBjYXNlcywgd2hlbiB2cS0+ bnVtX2FkZGVkIGlzIGJpZ2dlciwgb2xkIHdpbGwKPiBiZSBhIGJpZyBudW1iZXIuCj4KPiBCZXN0 IHJlZ2FyZHMsCj4gVGl3ZWkgQmllCj4KCkhvdyBhYm91dCBqdXN0IGRvIHNvbWV0aGluZyBsaWtl IHZob3N0OgoKc3RhdGljIHUxNiB2aG9zdF9pZHhfZGlmZihzdHJ1Y3Qgdmhvc3RfdmlydHF1ZXVl ICp2cSwgdTE2IG9sZCwgdTE2IG5ldykKewogwqDCoMKgIGlmIChuZXcgPiBvbGQpCiDCoMKgwqAg wqDCoMKgIHJldHVybiBuZXcgLSBvbGQ7CiDCoMKgwqAgcmV0dXJuwqAgKG5ldyArIHZxLT5udW0g LSBvbGQpOwp9CgpzdGF0aWMgYm9vbCB2aG9zdF92cmluZ19wYWNrZWRfbmVlZF9ldmVudChzdHJ1 Y3Qgdmhvc3RfdmlydHF1ZXVlICp2cSwKIMKgwqDCoCDCoMKgwqAgwqDCoMKgIMKgwqDCoCDCoMKg wqAgwqAgX191MTYgZXZlbnRfb2ZmLCBfX3UxNiBuZXcsCiDCoMKgwqAgwqDCoMKgIMKgwqDCoCDC oMKgwqAgwqDCoMKgIMKgIF9fdTE2IG9sZCkKewogwqDCoMKgIHJldHVybiAoX191MTYpKHZob3N0 X2lkeF9kaWZmKHZxLCBuZXcsIGV2ZW50X29mZikgLSAxKSA8CiDCoMKgwqAgwqDCoMKgwqDCoMKg IChfX3UxNil2aG9zdF9pZHhfZGlmZih2cSwgbmV3LCBvbGQpOwp9Cgo/Cl9fX19fX19fX19fX19f X19fX19fX19fX19fX19fX19fX19fX19fX19fX19fX19fClZpcnR1YWxpemF0aW9uIG1haWxpbmcg bGlzdApWaXJ0dWFsaXphdGlvbkBsaXN0cy5saW51eC1mb3VuZGF0aW9uLm9yZwpodHRwczovL2xp c3RzLmxpbnV4Zm91bmRhdGlvbi5vcmcvbWFpbG1hbi9saXN0aW5mby92aXJ0dWFsaXphdGlvbg==