linux-kernel - RE: [PATCH 1/1] scsi: storvsc: Enable scatterlist entry lengths

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Date:   Wed, 24 Feb 2021 17:03:59 +0000
From:   Michael Kelley <mikelley@...rosoft.com>
To:     Christoph Hellwig <hch@...radead.org>,
        vkuznets <vkuznets@...hat.com>
CC:     KY Srinivasan <kys@...rosoft.com>,
        "martin.petersen@...cle.com" <martin.petersen@...cle.com>,
        Long Li <longli@...rosoft.com>,
        "wei.liu@...nel.org" <wei.liu@...nel.org>,
        "jejb@...ux.ibm.com" <jejb@...ux.ibm.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-scsi@...r.kernel.org" <linux-scsi@...r.kernel.org>
Subject: RE: [PATCH 1/1] scsi: storvsc: Enable scatterlist entry lengths >
 4Kbytes

From: Christoph Hellwig <hch@...radead.org>  Sent: Wednesday, February 24, 2021 7:53 AM
> 
> Shouldn't storvsc just use blk_queue_virt_boundary instead of all this
> mess?

The storvsc driver does set the virt boundary to PAGE_SIZE - 1.  But
the driver still has to translate the scatterlist into a list of guest
physical frame numbers (each representing 4K bytes) that the
Hyper-V host understands so it can do the I/O.

This patch improves that translation so it can handle a single
scatterlist entry that represents more than PAGE_SIZE bytes of
data.  Then the SCSI dma_boundary (which turns into the blk level
segment_boundary) no longer needs to be set to restrict scatterlist
entries to just PAGE_SIZE bytes.

We also have to preserve the ability to run guests on ARM64 with
PAGE_SIZE of 16K or 64K, while Hyper-V still expects each PFN to
represent only 4K bytes.

Michael