[Xen-devel] Re: About block device mapping for guests

[Nuutti Kotivuori <naked@xxxxxx>]

M. A. Williamson wrote:
> The guest operating system (e.g. XenLinux) will bind the major /
> minor to a name according to it's own policy. e.g. under Linux 0x801
> is conventionally /dev/sda1 so unless you have a weird setup, that's
> what it'll appear as. I guess the files appear under SysFS similarly
> to how devices with the same major / minor would under vanilla
> Linux.

Well, I dug into this more deeply, since the argument seemed a bit
odd. To be able to convert 0x801 to a name, there has to be a mapping
table for it somewhere inside the kernel - and in vanilla linux, there
is none, since it is the driver that assigns both the sysfs name and
the major device number.

So I looked into the code and found this:

,----[ drivers/xen/blkfront/vbd.c ]
| static struct xlbd_type_info xlbd_ide_type = {
|        .partn_shift = 6,
|        .partn_per_major = 2,
|        // XXXcl todo blksize_size[major]  = 1024;
|        .hardsect_size = 512,
|        .max_sectors = 128,  /* 'hwif->rqsize' if we knew it */
|        // XXXcl todo read_ahead[major]    = 8; /* from 
drivers/ide/ide-probe.c */
|        .name = "hd",
| };
| 
| static struct xlbd_type_info xlbd_scsi_type = {
|        .partn_shift = 4,
|        .partn_per_major = 16,
|        // XXXcl todo blksize_size[major]  = 1024; /* XXX 512; */
|        .hardsect_size = 512,
|        .max_sectors = 128*8, /* XXX 128; */
|        // XXXcl todo read_ahead[major]    = 0; /* XXX 8; -- guessing */
|        .name = "sd",
| };
| 
| static struct xlbd_type_info xlbd_vbd_type = {
|        .partn_shift = 4,
|        .partn_per_major = 16,
|        // XXXcl todo blksize_size[major]  = 512;
|        .hardsect_size = 512,
|        .max_sectors = 128,
|        // XXXcl todo read_ahead[major]    = 8;
|        .name = "xvd",
| };
`----

And that answers all my remaining questions :-)

Thanks,
-- Naked



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