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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 14.03.2024 17:59, Jason Andryuk wrote:
> On 2024-03-14 11:30, Jan Beulich wrote:
>> 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.
> 
> Linux defines PHYS32_ENTRY with _ASM_PTR, so it's 32 or 64 bit to match 
> how the kernel is compiled.  The Xen code parses the integer according 
> to the size of the note.
> 
>>>> 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.
> 
> PVH is only implemented for x86 today.  Are you saying that the comment 
> should just specify the values are 32bit for x86?  If the note is reused 
> for other arches, then they can specify their usage?

Along these lines. But looks like Roger isn't concerned and would be
happy to leave that to the future.

> If this note is to be a variably sized array of values, then the 
> elements should be of fixed size.  Otherwise parsing is ambiguous 
> without, say, another field specifying element size.
> 
> Maybe XEN_ELFNOTE_PHYS32_RELOC would be a better name to complement the 
> PHYS32_ENTRY?

Perhaps, yes.

Jan