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Date: Fri, 5 May 2017 15:41:43 +0800
From: Fam Zheng <famz@redhat.com>
Message-ID: <20170505074143.GB21435@lemon.lan>
References: <20170426144645.12476-1-farman@linux.vnet.ibm.com>
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In-Reply-To: <20170426144645.12476-1-farman@linux.vnet.ibm.com>
Subject: Re: [Qemu-devel] [RFC PATCH v1 0/5] Enable virtio-scsi boot from
 /dev/sgX
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To: Eric Farman <farman@linux.vnet.ibm.com>
Cc: qemu-devel@nongnu.org, "Michael S . Tsirkin" <mst@redhat.com>, Alexander Graf <agraf@suse.de>, Christian Borntraeger <borntraeger@de.ibm.com>, Cornelia Huck <cornelia.huck@de.ibm.com>, Paolo Bonzini <pbonzini@redhat.com>

On Wed, 04/26 16:46, Eric Farman wrote:
> The short version of what happens is the host device driver rejects our
> requests because the transfer lengths are too long for it to satisfy.
> A virtio-scsi disk connected via scsi-generic is fine as a non-boot device
> because the guest kernel is able to break up the requests for us.  So we just
> need to handle this situation for the boot process.
> 
> Patches 2-N in this series do that, but rely on us to specify the max_sectors
> parameter for the virtio-scsi-ccw device:
> 
>   /usr/bin/qemu-system-s390x ...
>     -device virtio-scsi-ccw,id=scsi0,devno=fe.0.0001,max_sectors=2048

Can you instead do an INQUIRY from the bios code to check the Block Limits page?
The response is intercepted by hw/scsi/scsi-generic.c to merge in the host LUN's
limits. That's how Linux kernel finds the granularity for request splitting.

That way, patch 1 is not necessary too. I don't like it because it doesn't
always work considering LUN hotplug.

Fam