Re: [PATCH] Re: dom0 boot failure with 256G

[Mukesh Rathor <mukesh.rathor@oracle.com>]

Jan Beulich wrote:
>>>> Mukesh Rathor <mukesh.rathor@oracle.com> 06.06.08 04:43 >>>
>> Jan Beulich wrote:
>>>>   Trying to figure the hyp out, I notice phys addr is set as:
>>>>
>>>>   d->arch.physaddr_bitsize =
>>>>           fls((1UL << 32) - HYPERVISOR_COMPAT_VIRT_START(d)) - 1
>>>>             + (PAGE_SIZE - 2);
>>>>   which is set to 0x1019, totally baffling me. I'd expect it to be 32,
>>>>   36, or 64????????
>>> Of course it should be PAGE_SHIFT in here! c/s 14097 screwed this up
>>> - sorry, my fault.
>>>
>> Yup, that fixed it. Can you pl explain in couple sentences how we are getting 
>> at the physaddr_bitsize here?
> 
> The whole hypervisor hole is used solely for the (compatibility) m2p map
> when running on a 64-bit hypervisor. Hence the size of that hole, rounded
> down to a power of two, determines the address restriction a domain
> must be assigned. A kernel could theoretically specify being able to
> tolerate a larger hole (though XEN_ELFNOTE_HV_START_LOW), so to
> increase the physical address width it can get memory from.
> 
>> I still don't understand why 37 is an OK size for a 32-PAE guest (i thought 36 
>> was max).
> 
> 36 was the original max, but even on native hardware you can go beyond
> that nowadays. On 64-bit Xen, this was a natural extension even
> independent of confirming hardware support of wider than 36-bit physical
> addresses for 32-bit code.
> 
> Of course, we all realize that giving dom0 128G won't do any good, afair
> even native Linux can't reasonably deal with 64G (due to the huge
> memmap needed in lowmem). But allowing an as wide as possible range
> to get memory from for the domain is certainly very desirable (I am
> actually considering ideas on how to overcome even that limit, as in
> practical uses this is really undesirable to have, since there's no way
> to balloon out specific memory [i.e. such below the 37-bit address
> boundary] from other domains).
> 
> Jan

Got it. Thanks a lot, appreciate your time.

Mukesh