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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (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 A4A36C54E60 for ; Tue, 19 Mar 2024 06:39:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:Content-Type: Content-Transfer-Encoding:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:From:References:Cc:To:Subject: MIME-Version:Date:Message-ID:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=CYKClr9XvwHaU5CejY9BYVmbfUXxncJwpuotDmSl2ME=; b=MGywqDrMIud8HA Cm4ssoc7/GbYjBQsFE/bamD0bY2Gi12nJHaVJ1R0+sGcnYI1eF3QZTLTMzt2a5D9FlYuRgfDkDszy fvhzTEDjZuxlbFf7uJFQHk7POwWyjw1gPUu38/EGLd9k6B46MUGUxR59xIisaUabx+t0vnCa21hou iw2DnhcF5YBq15/ExZELkL2rQuOkwmgSb4vw8fbejloeLnRP6/q9HBc28Ht2AjaJpX8wDAC/jjov2 fAODFw0C2yAKTKmrd7z2n2qRIHVhEsWu7Icop4YatbQmnWVqQVYj6I1SwA2Z/thZydpXb4pOoUV3g wI/9sFKmZJnRiaF7Gj+w==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rmT7n-0000000BXBN-0otV; Tue, 19 Mar 2024 06:39:03 +0000 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by bombadil.infradead.org with esmtps (Exim 4.97.1 #2 (Red Hat Linux)) id 1rmT7l-0000000BXAQ-0bgs for linux-arm-kernel@lists.infradead.org; Tue, 19 Mar 2024 06:39:02 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1710830339; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=5PWFWfjNWiRH0RK0T1uriWMCNRFoVCxB1MnvhpCDvFQ=; b=A7cQnoxErC4BQEPeG4fDUZubt8vOjD1r0LI1eFbWhCGRYES5Aty3rnBGZZuT6ToItzqz0c RYjva08ML3jmcauxtoOC05OTHrbQ62vnJMYgXMXMOhGNVrWOGRqnFCIPDEXsvSaliKF0Zm UPEsx5gXBMjTHhEfpgB2zKN5LXWbUSg= Received: from mail-pg1-f198.google.com (mail-pg1-f198.google.com [209.85.215.198]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-310-EmI4HgIgOzeowi6M6w_V2w-1; Tue, 19 Mar 2024 02:38:56 -0400 X-MC-Unique: EmI4HgIgOzeowi6M6w_V2w-1 Received: by mail-pg1-f198.google.com with SMTP id 41be03b00d2f7-5cec090b2bdso3803999a12.0 for ; Mon, 18 Mar 2024 23:38:56 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1710830335; x=1711435135; h=content-transfer-encoding:in-reply-to:from:references:cc:to :content-language:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=5PWFWfjNWiRH0RK0T1uriWMCNRFoVCxB1MnvhpCDvFQ=; b=AhLaR71fLARSgVAdq/qLQ3mHl1EMOmgfEYQ13PR0CtZiEzmRWEmSRKobqbUxo05jj6 itqRdYlybBNcHa+LeWoKdVi9eUWI8R2T61YrPH1NAbj7z3LtUJ5pkJzvkwC5DYDcaotN Fpfd3jkZIEIAs7UfmpItEiGWcXumyNkhafHG59UPYBVEXs4TapCjOdE3O/TZzW52Iccj NaF6bBKyo/I/+WXwdnEJbELtpO5MX2OVd6a/inX0E9Pt+GiYkNnrMJGFxYTyTtFOXaKt hatW2fNhNxTSWnuzNnz7kTUSOm0RkrtQuZo3JnnBVq+D7+i6r5Pe6rIrGs+XiRu3nySK 52lA== X-Forwarded-Encrypted: i=1; AJvYcCU+lccZB0m5RV6O3SHRE2qEN3HZjPbsgVb/G9lQVQGuLkgQCeRWwOz/UuXnNvpKiH5Puo5zdxVfvBHujPEwz27lkaT04XSZUe2t60UwTCk3NPoexF8= X-Gm-Message-State: AOJu0Yx5YUnbo2kH0pRJ+0teZvn1eWosPwANBuFFr+O1rJ2rAJ4cnURf Da0oR8HoqA48kwIPCZe6XRWHZOSOV5yJlfAcfMblOgHbUqptrjRtxSDpns2yvIrdL9HJnha/YTs Qmp1UrYcSFswkSMpWdYEzTxDO+/CQWZKsOzY8ieDVbj4e/0xJZ8SPYIKkjOtEf5uw7MUFFtpk X-Received: by 2002:a05:6a21:78a9:b0:1a3:5299:1648 with SMTP id bf41-20020a056a2178a900b001a352991648mr11972195pzc.16.1710830335514; Mon, 18 Mar 2024 23:38:55 -0700 (PDT) X-Google-Smtp-Source: AGHT+IG4CuCVJiYhCSgYrUKlARZPvazBultLs5Ehqesd5coo9WIACzRgb9oUYr+Q8e3TOtm1pisQnQ== X-Received: by 2002:a05:6a21:78a9:b0:1a3:5299:1648 with SMTP id bf41-20020a056a2178a900b001a352991648mr11972179pzc.16.1710830335114; Mon, 18 Mar 2024 23:38:55 -0700 (PDT) Received: from [192.168.68.51] ([43.252.115.31]) by smtp.gmail.com with ESMTPSA id d6-20020a170902654600b001db8f7720e2sm10759114pln.288.2024.03.18.23.38.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 18 Mar 2024 23:38:54 -0700 (PDT) Message-ID: <9b3030d1-cb2c-4ce0-8b24-1074b616fc84@redhat.com> Date: Tue, 19 Mar 2024 16:38:49 +1000 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] virtio_ring: Fix the stale index in available ring To: "Michael S. Tsirkin" Cc: Will Deacon , virtualization@lists.linux.dev, linux-kernel@vger.kernel.org, jasowang@redhat.com, xuanzhuo@linux.alibaba.com, yihyu@redhat.com, shan.gavin@gmail.com, linux-arm-kernel@lists.infradead.org, Catalin Marinas , mochs@nvidia.com References: <20240314074923.426688-1-gshan@redhat.com> <20240318165924.GA1824@willie-the-truck> <35a6bcef-27cf-4626-a41d-9ec0a338fe28@redhat.com> <20240319020905-mutt-send-email-mst@kernel.org> From: Gavin Shan In-Reply-To: <20240319020905-mutt-send-email-mst@kernel.org> X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240318_233901_310981_B77E695B X-CRM114-Status: GOOD ( 19.37 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="us-ascii"; Format="flowed" Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 3/19/24 16:09, Michael S. Tsirkin wrote: >>>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c >>>> index 49299b1f9ec7..7d852811c912 100644 >>>> --- a/drivers/virtio/virtio_ring.c >>>> +++ b/drivers/virtio/virtio_ring.c >>>> @@ -687,9 +687,15 @@ static inline int virtqueue_add_split(struct virtqueue *_vq, >>>> avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); >>>> vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); >>>> - /* Descriptors and available array need to be set before we expose the >>>> - * new available array entries. */ >>>> - virtio_wmb(vq->weak_barriers); >>>> + /* >>>> + * Descriptors and available array need to be set before we expose >>>> + * the new available array entries. virtio_wmb() should be enough >>>> + * to ensuere the order theoretically. However, a stronger barrier >>>> + * is needed by ARM64. Otherwise, the stale data can be observed >>>> + * by the host (vhost). A stronger barrier should work for other >>>> + * architectures, but performance loss is expected. >>>> + */ >>>> + virtio_mb(false); >>>> vq->split.avail_idx_shadow++; >>>> vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, >>>> vq->split.avail_idx_shadow); >>> >>> Replacing a DMB with a DSB is _very_ unlikely to be the correct solution >>> here, especially when ordering accesses to coherent memory. >>> >>> In practice, either the larger timing different from the DSB or the fact >>> that you're going from a Store->Store barrier to a full barrier is what >>> makes things "work" for you. Have you tried, for example, a DMB SY >>> (e.g. via __smb_mb()). >>> >>> We definitely shouldn't take changes like this without a proper >>> explanation of what is going on. >>> >> >> Thanks for your comments, Will. >> >> Yes, DMB should work for us. However, it seems this instruction has issues on >> NVidia's grace-hopper. It's hard for me to understand how DMB and DSB works >> from hardware level. I agree it's not the solution to replace DMB with DSB >> before we fully understand the root cause. >> >> I tried the possible replacement like below. __smp_mb() can avoid the issue like >> __mb() does. __ndelay(10) can avoid the issue, but __ndelay(9) doesn't. >> >> static inline int virtqueue_add_split(struct virtqueue *_vq, ...) >> { >> : >> /* Put entry in available array (but don't update avail->idx until they >> * do sync). */ >> avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); >> vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); >> >> /* Descriptors and available array need to be set before we expose the >> * new available array entries. */ >> // Broken: virtio_wmb(vq->weak_barriers); >> // Broken: __dma_mb(); >> // Work: __mb(); >> // Work: __smp_mb(); >> // Work: __ndelay(100); >> // Work: __ndelay(10); >> // Broken: __ndelay(9); >> >> vq->split.avail_idx_shadow++; >> vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, >> vq->split.avail_idx_shadow); > > What if you stick __ndelay here? > /* Put entry in available array (but don't update avail->idx until they * do sync). */ avail = vq->split.avail_idx_shadow & (vq->split.vring.num - 1); vq->split.vring.avail->ring[avail] = cpu_to_virtio16(_vq->vdev, head); /* Descriptors and available array need to be set before we expose the * new available array entries. */ virtio_wmb(vq->weak_barriers); vq->split.avail_idx_shadow++; vq->split.vring.avail->idx = cpu_to_virtio16(_vq->vdev, vq->split.avail_idx_shadow); /* Try __ndelay(x) here as Michael suggested * * Work: __ndelay(200); possiblly make it hard to reproduce * Broken: __ndelay(100); * Broken: __ndelay(20); * Broken: __ndelay(10); */ __ndelay(200); > >> vq->num_added++; >> >> pr_debug("Added buffer head %i to %p\n", head, vq); >> END_USE(vq); >> : >> } >> >> I also tried to measure the consumed time for various barrier-relative instructions using >> ktime_get_ns() which should have consumed most of the time. __smb_mb() is slower than >> __smp_wmb() but faster than __mb() >> >> Instruction Range of used time in ns >> ---------------------------------------------- >> __smp_wmb() [32 1128032] >> __smp_mb() [32 1160096] >> __mb() [32 1162496] >> Thanks, Gavin _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel