Re: Still struggling to understand Xen

[Niels Dettenbach <nd@xxxxxxxxxxxx>]

Am Donnerstag, 2. Juli 2020, 14:00:25 CEST schrieb Biff Eros:
> Hypervisor in conjunction with dom0 provides disk and network devices
> on PCI busses that can be viewed, enumerated with standard
> off-the-shelf Linux drivers and tools.  This is all good.  My
> confusion kicks in when the subject of PV drivers comes up.
Just some parts in short:

While HVM virtualizes a "full platform" (like most heavyweight virt solutions), 
PV virtualizes ressources. 

This means:
In HVM, "standard drivers" could be used due to that full virtualization of 
hardware.

The reason behind: PV requires much less overhead just for "emulate" virtual 
"real" hardware on "real hardware" - it's main work is "just" providing 
"switching" ressources (transparently to the virtual devices in different DomU.

PV drivers like xen storage as xen net are presented to DomU ("guest") as the 
regarding devices over special drivers. I.e. you see no "buses" behind your 
storage device (disk or partition) nor have to deal with such as no access to 
devices on it without any special pv drivers. the hardware drivers are 
"covered" by xen / dom0 ("Host").


> It's also unclear to me if the back-end drivers in a typical dom0 that
> you might get from for example XCP-ng, or XenServer, or even AWS can
> somehow be incompatible with the latest and greatest domU Linux
> kernels.  Is there some kind of interface versioning or are all
> versions forward and backward compatible?
The DomU drivers "should" backward compatible (have some xen 3.x drivers 
anywhere on a much newer xen 4.x but that seems extreme - if i remember right 
there was some more significant change in some 3.x or so). They do so at least 
on my platforms.

If i'm right, these could be read with i.e. 

   xl info |grep xen_caps

xen_caps               : xen-3.0-x86_64 xen-3.0-x86_32p hvm-3.0-x86_32 
hvm-3.0-x86_32p hvm-3.0-x86_64


> I've been through pretty much all drivers related to Xen, compiled
> them into my kernel and selected /dev/xvda1 device on boot, but it's
> still not working for me, the Xen 'hardware' is not being detected, so
> would appreciate any guidance you can offer.

The device name seems not HVM, so i'm a bit confused.

Which devices are available at a DomU could be listed with:

   xl block-list <yourdomu>

and they should follow the naming of the xen storage driver within your DomU 
(may depend from DomU OS - i.e. Linux., *BSDs use different namings)

if it hangs "before" kernel boot: how do you boot your DomU (pygrub)?
if it hangs during boot (i.e. mounting root fs) show / grep kernel output 
regarding xen drivers ("blkfront", "xv")


niels.
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 Niels Dettenbach
 Syndicat IT & Internet
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