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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Thu, Mar 14, 2024 at 04:30:05PM +0100, 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.
> > 
> >> 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.

Elfnotes are used only on x86 so far.  I don't see why if/when another
architecture wants to use the same elfnotes names with different field
sizes that would be an issue.  When such a need arises we could
clarify that 32-bit size is only for x86 and also specify the size for
the other architecture.

Thanks, Roger.