From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DBA4F3346BC for ; Tue, 6 Jan 2026 14:51:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767711119; cv=none; b=dGar/ql/YIMua2tOIIg3FsFvffAv6oZhR96vD/SyhUXTp4Je8bgSBRQF3btt7VmdEOmDlg//BgstVN2cFATE2Mfsd6gBUKXBQu5fQdpJNiTVTKM1feJS6QMNzFitxNlZ+sfLDI3ydwHeHuh8SEl6Kjl9K2S8KWTZkatGHimd2P8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1767711119; c=relaxed/simple; bh=OtGN4S8+ZWGuAfn6+2YZp7eQlkz7c6TNLgozVUvUCR0=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: In-Reply-To:Content-Type:Content-Disposition; b=TR41pXA9vMZXbHqvzfUKQee85gZwm0iPt104fy4hzgfU22AkCEm7LYkbseBZA287/7U4Y6SrL1SK9XY8WuQA8aVh8LSjcUNziX6OnbwEhqRX2gl/a9elAegXB9WeLFIEe+bAoDIADsoeWYP4b2Mr9klSl/sUxbNKn8sn7EKGpwk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=BL/teLkI; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="BL/teLkI" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1767711116; 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: in-reply-to:in-reply-to:references:references; bh=v8xGpc9e0AHSzqXJdY4dTy2UmvNhNHIufeUOIUm/LzQ=; b=BL/teLkIPThhL9jHSrLxfnXBDAY94IA+4vZjWBnQV3yo45uDf/RAL3O+P6o/grWXBJUjBy kSdVyRT+DYU1jyL9Eh3w4KbjJ22QUBmyP4Qj/RE/9lA4/4ta+qMswvZeeitZXTT4Xgm/pF VPPkgscRwkHWKhiljX3vgwBG56z+dwE= Received: from mail-wr1-f69.google.com (mail-wr1-f69.google.com [209.85.221.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-641-LUmfE0v7N92EfFh7cfnlcw-1; Tue, 06 Jan 2026 09:51:53 -0500 X-MC-Unique: LUmfE0v7N92EfFh7cfnlcw-1 X-Mimecast-MFC-AGG-ID: LUmfE0v7N92EfFh7cfnlcw_1767711112 Received: by mail-wr1-f69.google.com with SMTP id ffacd0b85a97d-4310062d97bso687250f8f.0 for ; Tue, 06 Jan 2026 06:51:53 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1767711112; x=1768315912; h=in-reply-to:content-disposition:mime-version:references:message-id :subject:cc:to:from:date:x-gm-gg:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=v8xGpc9e0AHSzqXJdY4dTy2UmvNhNHIufeUOIUm/LzQ=; b=teeJTPq6G08H8heXJtArRXggDT0nOnfFEIhVeXY5dpMCPKQNRqEAwso77DhEPlXacL FCfXetLJf4JFjZ1AKaPpRjYfxKsVVjPPQnYFCkVI9lSlq5uBUNJChrjUxjGSl2FcOjt/ LhAMqlgEGuAem5iFiFh+1DtlP0bzt28LMLDHEGpq4gRQfiiAhL891QHZs4Npazkgd3bA kgBajRejCRvgrPjfyb1RwHpKyGE5Ead2wrvJUQgchXY+ptK46Ulls4AfsGepCwQZvai4 RVx7F8f0abn2a5LaKulOshGBhXJQvsViNhS2YQRxuxPsVrokQquipW+WlKj1lJVL20Vk QqAw== X-Forwarded-Encrypted: i=1; AJvYcCUoyUK/JAbznTYYDYNZl3ZvU1IQjWp7FmuNKHQddGAlxAOdsex8ngvqfK8NSd0b9hjQ2dknPQ==@lists.linux.dev X-Gm-Message-State: AOJu0YyXlufLbHwFrasKdKssXFfmw/84F14FJoAIptxRcFBhvsLOTaQ8 P2mV9fDvcdhZIGMacaakEqpeRLs7AnFoxvkLGPFAkcOmMn6JuRMm7S1mx7zH+6UQfNNhdyuBRJY 7eTRyXmHrM4Azw+1i8JEdzC8yK72Cmbpi8KSgpZo4U/LbDO5pynoFgGrR X-Gm-Gg: AY/fxX6gVykjm4aR8Yi2r6NBmsD9jfuvRDGGa7cg2lF1LY7pF5iigqD3ARnsdi2f7mz bPvMSe6CLrL8shSzh4O1Aa796w2j4vJouYKMv4LWSu7Cc/bVfNHIrX9x2s+TRyAIDZUjGtwn23I nXFYaR0OmBeTmvVTMlUX0dpDVZ4PUyP+N4UuCJDngv7wn/KpRFWIhqxUSc9yEWYt5F8f+zZd/nz p2yi8b3CG3fRWDpu7qDGRKma5a/ILft91JBtX+Cj238ovJnO2Uue90fjNG48rPpH+RKwAobJ8I1 Q5X9o9YnNUlnZhQEVkdcgbezOdi5ZjkTukX2/BIZPjnTbbmMBBgbdrz2Qx+hize+aBWTqWHK4cA OsoSAr0R8piLGNrsb83x0As0k1GiiByEr4Q== X-Received: by 2002:a05:6000:200f:b0:431:b6e:8be3 with SMTP id ffacd0b85a97d-432bc9f6dccmr4753188f8f.38.1767711112111; Tue, 06 Jan 2026 06:51:52 -0800 (PST) X-Google-Smtp-Source: AGHT+IGOynDA9QP29UwN/sUhG6OCzOdNXrD0Y6rHdaimQHcJZW2G3EKin+vfxrhBk6BDORYTgSmwkg== X-Received: by 2002:a05:6000:200f:b0:431:b6e:8be3 with SMTP id ffacd0b85a97d-432bc9f6dccmr4753140f8f.38.1767711111656; Tue, 06 Jan 2026 06:51:51 -0800 (PST) Received: from redhat.com (IGLD-80-230-31-118.inter.net.il. [80.230.31.118]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-432bd5ede7esm4752453f8f.32.2026.01.06.06.51.48 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 06 Jan 2026 06:51:51 -0800 (PST) Date: Tue, 6 Jan 2026 09:51:46 -0500 From: "Michael S. Tsirkin" To: Stefan Hajnoczi Cc: linux-kernel@vger.kernel.org, Cong Wang , Jonathan Corbet , Olivia Mackall , Herbert Xu , Jason Wang , Paolo Bonzini , Eugenio =?iso-8859-1?Q?P=E9rez?= , "James E.J. Bottomley" , "Martin K. Petersen" , Gerd Hoffmann , Xuan Zhuo , Marek Szyprowski , Robin Murphy , Stefano Garzarella , "David S. Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Petr Tesarik , Leon Romanovsky , Jason Gunthorpe , Bartosz Golaszewski , linux-doc@vger.kernel.org, linux-crypto@vger.kernel.org, virtualization@lists.linux.dev, linux-scsi@vger.kernel.org, iommu@lists.linux.dev, kvm@vger.kernel.org, netdev@vger.kernel.org Subject: Re: [PATCH v2 10/15] virtio_scsi: fix DMA cacheline issues for events Message-ID: <20260106095044-mutt-send-email-mst@kernel.org> References: <8801aeef7576a155299f19b6887682dd3a272aba.1767601130.git.mst@redhat.com> <20260105181939.GA59391@fedora> Precedence: bulk X-Mailing-List: iommu@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 In-Reply-To: <20260105181939.GA59391@fedora> X-Mimecast-Spam-Score: 0 X-Mimecast-MFC-PROC-ID: dDQeUwv-4brx17vBNnBUM95qZoz_Bjo4rCJgy7ClGAU_1767711112 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline On Mon, Jan 05, 2026 at 01:19:39PM -0500, Stefan Hajnoczi wrote: > On Mon, Jan 05, 2026 at 03:23:29AM -0500, Michael S. Tsirkin wrote: > > @@ -61,7 +62,7 @@ struct virtio_scsi_cmd { > > > > struct virtio_scsi_event_node { > > struct virtio_scsi *vscsi; > > - struct virtio_scsi_event event; > > + struct virtio_scsi_event *event; > > struct work_struct work; > > }; > > > > @@ -89,6 +90,11 @@ struct virtio_scsi { > > > > struct virtio_scsi_vq ctrl_vq; > > struct virtio_scsi_vq event_vq; > > + > > + __dma_from_device_group_begin(); > > + struct virtio_scsi_event events[VIRTIO_SCSI_EVENT_LEN]; > > + __dma_from_device_group_end(); > > If the device emits two events in rapid succession, could the CPU see > stale data for the second event because it already holds the cache line > for reading the first event? > > In other words, it's not obvious to me that the DMA warnings are indeed > spurious and should be silenced here. > > It seems safer and simpler to align and pad the struct virtio_scsi_event > field in struct virtio_scsi_event_node rather than packing these structs > into a single array here they might share cache lines. > > Stefan To add to what I wrote, that's a lot of overhead: 8 * 128 - about 1K on some platforms, and these happen to be low end ones. -- MST