Re: [PATCH v2 3/3] x86/PVH: Support relocatable dom0 kernels

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2024 15:33, Roger Pau Monné wrote:
> On Thu, Mar 14, 2024 at 09:51:22AM -0400, Jason Andryuk wrote:
>> On 2024-03-14 05:48, Roger Pau Monné wrote:
>>> On Wed, Mar 13, 2024 at 03:30:21PM -0400, Jason Andryuk wrote:
>>>> @@ -234,6 +235,17 @@ elf_errorstatus elf_xen_parse_note(struct elf_binary 
>>>> *elf,
>>>>                   elf_note_numeric_array(elf, note, 8, 0),
>>>>                   elf_note_numeric_array(elf, note, 8, 1));
>>>>           break;
>>>> +
>>>> +    case XEN_ELFNOTE_PVH_RELOCATION:
>>>> +        if ( elf_uval(elf, note, descsz) != 3 * sizeof(uint64_t) )
>>>> +            return -1;
>>>> +
>>>> +        parms->phys_min = elf_note_numeric_array(elf, note, 8, 0);
>>>> +        parms->phys_max = elf_note_numeric_array(elf, note, 8, 1);
>>>> +        parms->phys_align = elf_note_numeric_array(elf, note, 8, 2);
>>>
>>> Size for those needs to be 4 (32bits) as the entry point is in 32bit
>>> mode?  I don't see how we can start past the 4GB boundary.
>>
>> I specified the note as 3x 64bit values.  It seemed simpler than trying to
>> support both 32bit and 64bit depending on the kernel arch.  Also, just using
>> 64bit provides room in case it is needed in the future.
> 
> Why do you say depending on the kernel arch?
> 
> PVH doesn't know the bitness of the kernel, as the kernel entry point
> is always started in protected 32bit mode.  We should just support
> 32bit values, regardless of the kernel bitness, because that's the
> only range that's suitable in order to jump into the entry point.
> 
> Note how XEN_ELFNOTE_PHYS32_ENTRY is also unconditionally a 32bit
> integer.
> 
>> Do you want the note to be changed to 3x 32bit values?
> 
> Unless anyone objects, yes, that's would be my preference.

As mentioned elsewhere, unless the entire note is meant to be x86-specific,
this fixed-32-bit property then would want limiting to x86.

Jan