From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 97C74CD37B0 for ; Mon, 18 Sep 2023 12:30:10 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1qiDNi-0007SW-Ep; Mon, 18 Sep 2023 08:29:39 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qiDNg-0007Qa-Aa for qemu-devel@nongnu.org; Mon, 18 Sep 2023 08:29:36 -0400 Received: from frasgout.his.huawei.com ([185.176.79.56]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1qiDNc-0006R2-0a for qemu-devel@nongnu.org; Mon, 18 Sep 2023 08:29:35 -0400 Received: from lhrpeml500005.china.huawei.com (unknown [172.18.147.207]) by frasgout.his.huawei.com (SkyGuard) with ESMTP id 4Rq3xM18sLz6K6WS; Mon, 18 Sep 2023 20:28:39 +0800 (CST) Received: from localhost (10.202.227.76) by lhrpeml500005.china.huawei.com (7.191.163.240) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256) id 15.1.2507.31; Mon, 18 Sep 2023 13:29:27 +0100 Date: Mon, 18 Sep 2023 13:29:26 +0100 To: Michael Tokarev CC: , Michael Tsirkin , Fan Ni , Philippe =?ISO-8859-1?Q?Mathieu-Daud=E9?= , Subject: Re: [PATCH 2/4] hw/cxl: Use available size parameter to index into register arrays. Message-ID: <20230918132926.00005c97@Huawei.com> In-Reply-To: References: <20230913150521.30035-1-Jonathan.Cameron@huawei.com> <20230913150521.30035-3-Jonathan.Cameron@huawei.com> Organization: Huawei Technologies Research and Development (UK) Ltd. X-Mailer: Claws Mail 4.1.0 (GTK 3.24.33; x86_64-w64-mingw32) MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-Originating-IP: [10.202.227.76] X-ClientProxiedBy: lhrpeml100001.china.huawei.com (7.191.160.183) To lhrpeml500005.china.huawei.com (7.191.163.240) X-CFilter-Loop: Reflected Received-SPF: pass client-ip=185.176.79.56; envelope-from=jonathan.cameron@huawei.com; helo=frasgout.his.huawei.com X-Spam_score_int: -41 X-Spam_score: -4.2 X-Spam_bar: ---- X-Spam_report: (-4.2 / 5.0 requ) BAYES_00=-1.9, RCVD_IN_DNSWL_MED=-2.3, RCVD_IN_MSPIKE_H5=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Reply-to: Jonathan Cameron From: Jonathan Cameron via Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Thu, 14 Sep 2023 15:54:54 +0300 Michael Tokarev wrote: > 13.09.2023 18:05, Jonathan Cameron via wrote: > > Indexing has to be done into an array with the right size elements. > > As such, the size parameter always matches the array element size > > and can be used in place of the longer sizeof(*array) > > > > Signed-off-by: Jonathan Cameron > > --- > > hw/cxl/cxl-component-utils.c | 8 ++++---- > > 1 file changed, 4 insertions(+), 4 deletions(-) > > > > diff --git a/hw/cxl/cxl-component-utils.c b/hw/cxl/cxl-component-utils.c > > index f3bbf0fd13..089e10b232 100644 > > --- a/hw/cxl/cxl-component-utils.c > > +++ b/hw/cxl/cxl-component-utils.c > > @@ -76,7 +76,7 @@ static uint64_t cxl_cache_mem_read_reg(void *opaque, hwaddr offset, > > if (cregs->special_ops && cregs->special_ops->read) { > > return cregs->special_ops->read(cxl_cstate, offset, size); > > } else { > > - return cregs->cache_mem_registers[offset / sizeof(*cregs->cache_mem_registers)]; > > + return cregs->cache_mem_registers[offset / size]; > > This is a though one, and smells wrong. > > Though because it is not at all obvious where this "size" value comes from, > have to find usage(s) of this function (cache_mem_ops) and think twice about > the other parameters in there. Also having in mind the previous comparison > with 8. In this part of the code, size should always be =4, but it takes > hard time to figure this out. > > Wrong - no, because of the above - the only 2 possible values are 4 and 8, > but it's difficult to see what's going on, and you're making it worse. > > Original code was at least clear you're getting a single register from > an array of registers, with new code it is not clear at all. Fair point. > > What I'd probably use here is to add comment that size can be either 4 or 8, > and use a switch similar to what you've used in first patch in this series. > And have a static_assert(sizeof(register) == 4) or something like that > here in this second branch. Good idea. > > So it is something like: > > static uint64_t cxl_cache_mem_read_reg(void *opaque, hwaddr offset, > unsigned size) > { > CXLComponentState *cxl_cstate = opaque; > ComponentRegisters *cregs = &cxl_cstate->crb; > > switch (size) { > case 8: > qemu_log_mask(LOG_UNIMP, > "CXL 8 byte cache mem registers not implemented\n"); > return 0; > > case 4: > if (cregs->special_ops && cregs->special_ops->read) { > return cregs->special_ops->read(cxl_cstate, offset, 4); > } else { > return cregs->cache_mem_registers[offset / > sizeof(*cregs->cache_mem_registers)]; > } > > default: > /* this routine is called with size being either 4 or 8 only */ > g_assert_not_reached(); > } > } > > Note: I especially left the sizeof() here, instead of using a previously > suggested static_assert() - because a register can be implemented using > larger integers on the host, it does not need to be 4 bytes, - but only > low 4 bytes can actually be used. I don't follow. Here cache_mem_registers is an array of uint32_t so it is going to be 4 bytes on any host! Obviously that's not true for registers in general. So I've added the assert as it is always valid and made it a / 4 Note I ended up with just using size in the first place because I was planning to add a local variable that was always the same size and that was silly. > > This does not shorten the line (it does by wrapping it up), but it keep > code correct and more understandable. Adding size parameter there makes > it much more cryptic. > > Here and in other places. > > This is just an example, not a suggestion. It makes sense. Sorry - thought I'd sent this last week! Jonathan > > /mjt >