linux-kernel - Re: [PATCH 10/13] megaraid_sas: set virt_boundary

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Date:   Mon, 17 Jun 2019 10:44:33 +0200
From:   Christoph Hellwig <hch@....de>
To:     Kashyap Desai <kashyap.desai@...adcom.com>
Cc:     Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Sebastian Ott <sebott@...ux.ibm.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Max Gurtovoy <maxg@...lanox.com>,
        Bart Van Assche <bvanassche@....org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Alan Stern <stern@...land.harvard.edu>,
        Oliver Neukum <oneukum@...e.com>, linux-block@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-mmc@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-scsi@...r.kernel.org,
        "PDL,MEGARAIDLINUX" <megaraidlinux.pdl@...adcom.com>,
        PDL-MPT-FUSIONLINUX <mpt-fusionlinux.pdl@...adcom.com>,
        linux-hyperv@...r.kernel.org, linux-usb@...r.kernel.org,
        usb-storage@...ts.one-eyed-alien.net, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/13] megaraid_sas: set virt_boundary_mask in the scsi
 host

On Fri, Jun 14, 2019 at 01:28:47AM +0530, Kashyap Desai wrote:
> Is there any changes in API  blk_queue_virt_boundary? I could not find
> relevant code which account for this. Can you help ?
> Which git repo shall I use for testing ? That way I can confirm, I didn't
> miss relevant changes.

Latest mainline plus the series (which is about to get resent).
blk_queue_virt_boundary now forced an unlimited max_hw_sectors as that
is how PRP-like schemes work, to work around a block driver merging
bug.  But we also need to communicate that limit to the DMA layer so
that we don't set a smaller iommu segment size limitation.

> >From your above explanation, it means (after this patch) max segment size
> of the MR controller will be set to 4K.
> Earlier it is possible to receive single SGE of 64K datalength (Since max
> seg size was 64K), but now the same buffer will reach the driver having 16
> SGEs (Each SGE will contain 4K length).

No, there is no more limit for the size of the segment at all,
as for PRPs each PRP is sort of a segment from the hardware perspective.
We just require the segments to not have gaps, as PRPs don't allow for
that.

That being said I think these patches are wrong for the case of megaraid
or mpt having both NVMe and SAS/ATA devices behind a single controller.
Is that a valid configuration?