From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <SRS0=drEb=P7=vger.kernel.org=linux-kernel-owner@kernel.org>
X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on
	aws-us-west-2-korg-lkml-1.web.codeaurora.org
X-Spam-Level: 
X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,
	MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=unavailable
	autolearn_force=no version=3.4.0
Received: from mail.kernel.org (mail.kernel.org [198.145.29.99])
	by smtp.lore.kernel.org (Postfix) with ESMTP id 4D5FDC282C2
	for <linux-kernel@archiver.kernel.org>; Wed, 23 Jan 2019 21:31:44 +0000 (UTC)
Received: from vger.kernel.org (vger.kernel.org [209.132.180.67])
	by mail.kernel.org (Postfix) with ESMTP id 261CD218A1
	for <linux-kernel@archiver.kernel.org>; Wed, 23 Jan 2019 21:31:44 +0000 (UTC)
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1726881AbfAWVbm (ORCPT
        <rfc822;linux-kernel@archiver.kernel.org>);
        Wed, 23 Jan 2019 16:31:42 -0500
Received: from verein.lst.de ([213.95.11.211]:45040 "EHLO newverein.lst.de"
        rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP
        id S1726352AbfAWVbl (ORCPT <rfc822;linux-kernel@vger.kernel.org>);
        Wed, 23 Jan 2019 16:31:41 -0500
Received: by newverein.lst.de (Postfix, from userid 2407)
        id C43B768CEC; Wed, 23 Jan 2019 22:31:39 +0100 (CET)
Date:   Wed, 23 Jan 2019 22:31:39 +0100
From:   Christoph Hellwig <hch@lst.de>
To:     Joerg Roedel <joro@8bytes.org>
Cc:     "Michael S . Tsirkin" <mst@redhat.com>,
        Jason Wang <jasowang@redhat.com>,
        Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
        Christoph Hellwig <hch@lst.de>, Jens Axboe <axboe@kernel.dk>,
        virtualization@lists.linux-foundation.org,
        linux-block@vger.kernel.org, linux-kernel@vger.kernel.org,
        iommu@lists.linux-foundation.org, jfehlig@suse.com,
        jon.grimm@amd.com, brijesh.singh@amd.com, jroedel@suse.de
Subject: Re: [PATCH 5/5] virtio-blk: Consider virtio_max_dma_size() for
 maximum segment size
Message-ID: <20190123213139.GD9032@lst.de>
References: <20190123163049.24863-1-joro@8bytes.org> <20190123163049.24863-6-joro@8bytes.org>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20190123163049.24863-6-joro@8bytes.org>
User-Agent: Mutt/1.5.17 (2007-11-01)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Wed, Jan 23, 2019 at 05:30:49PM +0100, Joerg Roedel wrote:
> +	max_size = virtio_max_dma_size(vdev);
> +
>  	/* Host can optionally specify maximum segment size and number of
>  	 * segments. */
>  	err = virtio_cread_feature(vdev, VIRTIO_BLK_F_SIZE_MAX,
>  				   struct virtio_blk_config, size_max, &v);
>  	if (!err)
> -		blk_queue_max_segment_size(q, v);
> -	else
> -		blk_queue_max_segment_size(q, -1U);
> +		max_size = min(max_size, v);
> +
> +	blk_queue_max_segment_size(q, max_size);

I wonder if we should just move the dma max segment size check
into blk_queue_max_segment_size so that all block drivers benefit
from it.  Even if not I think at least the SCSI midlayer should
be updated to support it.

Btw, I wonder why virtio-scsi sticks to the default segment size,
unlike virtio-blk.