Re: [Xen 4.12 bug] HVM/PVH boot confusion

[Wei Liu <wei.liu2@citrix.com>]

On Tue, Jan 29, 2019 at 07:49:51PM +0000, Andrew Cooper wrote:
> Hello,
> 
> Given the following vm.cfg file:
> 
> name="vm"
> type="hvm"
> 
> vcpus=4
> memory=1024
> 
> firmware_override="/root/xen-syms"
> 
> kernel="/boot/vmlinuz-4.4-xen"
> ramdisk="/boot/initrd-4.4.0+10.img"
> 
> cmdline="console=xen,pv dom0=pv --- earlyprintk=xen"
> 

What is your use case here? If you want to use the shim, why don't you
put pvshim=1 directly?

> Xen crashes with the following trace:
> 
> (d15) (XEN) Xen BUG at pvh-boot.c:82
> (d15) (XEN) ----[ Xen-4.12.0-rc  x86_64  debug=y   Not tainted ]----
> (d15) (XEN) CPU:    0
> (d15) (XEN) RIP:    e008:[<ffff82d0804331f2>] pvh_init+0x27d/0x2fe
> <snip>
> (d15) (XEN) Xen call trace:
> (d15) (XEN)    [<ffff82d0804331f2>] pvh_init+0x27d/0x2fe
> (d15) (XEN)    [<ffff82d080429000>] __start_xen+0x14c/0x28f6
> (d15) (XEN)    [<ffff82d0802000f3>] __high_start+0x53/0x55
> (d15) (XEN)
> (d15) (XEN)
> (d15) (XEN) ****************************************
> (d15) (XEN) Panic on CPU 0:
> (d15) (XEN) Xen BUG at pvh-boot.c:82
> (d15) (XEN) ****************************************
> 
> The problem is that Xen is started at its PVH entrypoint (contrary to
> the instructions in the vm config file), and Xen unconditionally expects
> RSDP to be passed.
> 
> There are at least two bugs here.
> 
> 1) RSDP was a late addition to the PVH boot protocol.  Xen's PVH
> entrypoint must not mandate its existence, because there are releases of
> the domain builder which don't provide it.

The inner Xen, in this case, can fall back to scanning low 1MB memory
for RSDP.

> 
> 2) The HVM/PVH boot confusion.  This think this is a still-outstanding
> bug around the broken assumption that the hvmloader binary speaks the
> PVH protocol without advertising itself appropriately (I really regret
> not objecting to those patches before they went in).  At the least, that
> needs fixing by putting a proper ELF note in hvmloader, and the domain
> builder needs to be updated to build all PVH-boot-ABI images consistently.

Do you expect users to drop an arbitrary hvmloader into an arbitrary
version of Xen and they continue to work?

Wei.

> 
> I don't have time to look into this at the moment, if anyone fancies
> trying to fix these issues.
> 
> ~Andrew

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