Re: [PATCH v3 13/28] hw/xen: automatically assign device index to block devices

From: "Durrant, Paul" <xadimgnik@gmail.com>
To: David Woodhouse <dwmw2@infradead.org>,
	paul@xen.org, qemu-devel@nongnu.org
Cc: "Kevin Wolf" <kwolf@redhat.com>,
	"Hanna Reitz" <hreitz@redhat.com>,
	"Stefano Stabellini" <sstabellini@kernel.org>,
	"Anthony Perard" <anthony.perard@citrix.com>,
	"Marc-André Lureau" <marcandre.lureau@redhat.com>,
	"Paolo Bonzini" <pbonzini@redhat.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	"Marcel Apfelbaum" <marcel.apfelbaum@gmail.com>,
	"Richard Henderson" <richard.henderson@linaro.org>,
	"Eduardo Habkost" <eduardo@habkost.net>,
	"Jason Wang" <jasowang@redhat.com>,
	"Marcelo Tosatti" <mtosatti@redhat.com>,
	qemu-block@nongnu.org, xen-devel@lists.xenproject.org,
	kvm@vger.kernel.org, "Bernhard Beschow" <shentey@gmail.com>,
	"Joel Upham" <jupham125@gmail.com>
Subject: Re: [PATCH v3 13/28] hw/xen: automatically assign device index to block devices
Date: Fri, 27 Oct 2023 10:01:06 +0100	[thread overview]
Message-ID: <9fb67e52-f262-4cf4-91c2-a42411ba21c4@gmail.com> (raw)
In-Reply-To: <f72e2e7feed3ecf17af8ab8442c359eea329ef17.camel@infradead.org>

On 27/10/2023 09:45, David Woodhouse wrote:
> On Fri, 2023-10-27 at 08:30 +0100, Durrant, Paul wrote:
>>
>>> +    if (blockdev->props.vdev.type == XEN_BLOCK_VDEV_TYPE_INVALID) {
>>> +        XenBus *xenbus = XEN_BUS(qdev_get_parent_bus(DEVICE(xendev)));
>>> +        char fe_path[XENSTORE_ABS_PATH_MAX + 1];
>>> +        char *value;
>>> +        int disk = 0;
>>> +        unsigned long idx;
>>> +
>>> +        /* Find an unoccupied device name */
>>
>> Not sure this is going to work is it? What happens if 'hda' or 'sda', or
>> 'd0' exists? I think you need to use the core of the code in
>> xen_block_set_vdev() to generate names and search all possible encodings
>> for each disk.
> 
> Do we care? You're allowed to have *all* of "hda", "sda" and "xvda" at
> the same time. If a user explicitly provides "sda" and then provides
> another disk without giving it a name, we're allowed to use "xvda".
> 

Maybe sda and xvda can co-exist, but

https://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=docs/man/xen-vbd-interface.7.pandoc;h=ba0d159dfa7eaf359922583ccd6d2b413acddb13;hb=HEAD#l125

says that you'll likely run into trouble if hda exists and you happen to 
create xvda.

> Hell, you can also have *separate* backing stores provided as "hda1",
> "sda1" and "xvda1". I *might* have tolerated a heckle that this
> function should check for at least the latter of those, but when I was
> first coding it up I was more inclined to argue "Don't Do That Then".

This code is allocating a name automatically so I think the onus is on 
it not create a needless clash which is likely to have unpredictable 
results depending on what the guest is. Just avoid any aliasing in the 
first place and things will be fine.

   Paul