Re: [PATCH 0/3] x86/pvh: Support relocating dom0 kernel

[Jan Beulich <jbeulich@xxxxxxxx>]

On 07.03.2024 18:33, Jason Andryuk wrote:
> On 2024-03-07 05:20, Roger Pau Monné wrote:
>> On Thu, Mar 07, 2024 at 11:08:37AM +0100, Jan Beulich wrote:
>>> On 07.03.2024 11:00, Roger Pau Monné wrote:
>>>> On Wed, Mar 06, 2024 at 01:50:29PM -0500, Jason Andryuk wrote:
>>>>> Xen tries to load a PVH dom0 kernel at the fixed guest physical address
>>>>> from the elf headers.  For Linux, this defaults to 0x1000000 (16MB), but
>>>>> it can be configured.
>>>>>
>>>>> Unfortunately there exist firmwares that have reserved regions at this
>>>>> address, so Xen fails to load the dom0 kernel since it's not RAM.
>>>>>
>>>>> The other issue is that the Linux PVH entry point is not
>>>>> position-independent.  It expects to run at the compiled
>>>>> CONFIG_PHYSICAL_ADDRESS.
>>>>>
>>>>> This patch set expands the PVH dom0 builder to try to relocate the
>>>>> kernel if needed and possible.  XENFEAT_pvh_relocatable is added for
>>>>> kernels to indicate they are relocatable.  However, we may want to
>>>>> switch to an additional ELF note with the kernel alignment.  Linux
>>>>> specifies a kernel alignment in the bzImage boot_params.setup_header,
>>>>> but that is not present the ELF vmlinux file.
>>>>
>>>> I wonder whether we need a pair of notes, to signal the min/max
>>>> addresses the kernel supports being relocated to.
>>>
>>> Plus, as per your other reply, a 3rd one to specify alignment needs.
>>
>> Alignment we could in theory get from the ELF program header, if OSes
>> fill those reliably.  FreeBSD seems to do so, haven't checked Linux.
> 
> I will look into this more, but at first glance, I don't see a 
> connection between Linux's CONFIG_PHYSICAL_ALIGN and the values in the 
> elf headers.  As a quick test, I set CONFIG_PHYSICAL_ALIGN=0x800000, but 
> the elf align values are still 0x200000.
> 
> The elf header values may be a suitable fallback though?

Imo, given the above, explicit values should be required. Better not
load a kernel than doing so and then getting hard to debug crashes.

Jan