Re: [Xen-devel] [Draft A] Boot ABI for HVM guests without a device-model

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 10.06.15 at 20:55, <konrad.wilk@xxxxxxxxxx> wrote:
> On Wed, Jun 10, 2015 at 02:34:00PM +0200, Roger Pau Monnà wrote:
>> The first three notes contain information about the guest kernel and 
>> the Xen hypercall ABI version. The following notes are of special 
>> interest:
>> 
>>  * XEN_ELFNOTE_PADDR_OFFSET: the offset of the ELF paddr field from the
>>    actual required physical address.
>>  * XEN_ELFNOTE_PADDR_ENTRY: the 32bit entry point into the kernel.
> 
> Is 'P' suppose to be 'physical' ?
> 
> I am not sure how this will work with an ELF 64-bit binary like
> the Linux kernel. Usually we use the virtual address but with
> us starting in 32-bit mode with an 64-bit virtual address won't work.

So first you correctly decode the 'P' as physical, and then you're
concerned about _virtual_ addresses? The Linux ELF PHDR has
perfectly valid virtual _and_ physical addresses in it afaict.

>> Note that the boot protocol resembles the multiboot1 specification, 
>> this is done so OSes with multiboot1 entry points can reuse those if 
>> desired. Also note that the processor starts with paging disabled, 
>> which means that all the memory addresses in the start_info page will 
>> be physical memory addresses.
> 
> Wow?! Pagetables disabled?! Why? Usually boot loaders start with some
> pagetables setup for the OS - to cover at least the kernel and the
> ramdisk. Either it being in 1-1 pagetables or such.

Mind pointing out which boot loaders you think about here? Both
multiboot variants surely start the OS in non-paged protected
mode. Of course, UEFI is completely different (because it wants
itself to run in 64-bit mode).

Jan

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