Re: [RFC] xen/pvh: detect PVH after kexec

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 03/21/2017 10:44 AM, Vitaly Kuznetsov wrote:
> Vitaly Kuznetsov <vkuznets@redhat.com> writes:
>
>> Roger Pau Monne <roger.pau@citrix.com> writes:
>>
>>> On Tue, Mar 21, 2017 at 10:21:52AM +0100, Vitaly Kuznetsov wrote:
>>>> Boris Ostrovsky <boris.ostrovsky@oracle.com> writes:
>>>>
>>>>> On 03/20/2017 02:20 PM, Vitaly Kuznetsov wrote:
>>>>>> PVH guests after kexec boot like normal HVM guests and we're not entering
>>>>>> xen_prepare_pvh()
>>>>> Is it not? Aren't we going via xen_hvm_shutdown() and then
>>>>> SHUTDOWN_soft_reset which would restart at the same entry point as
>>>>> regular boot?
>>>> No, we're not doing regular boot: from outside of the guest we don't
>>>> really know where the new kernel is placed (as guest does it on its
>>>> own). We do soft reset to clean things up and then guest jumps to the
>>>> new kernel starting point by itself.
>>>>
>>>> We could (in theory, didn't try) make it jump to the PVH starting point
>>>> but we'll have to at least prepare the right boot params for
>>>> init_pvh_bootparams and this looks like additional
>>>> complication. PVHVM-style startup suits us well but we still need to be
>>>> PVH-aware.
>>> We are going to have the same issue when booting PVH with OVMF, Linux will be
>>> started at the native UEFI entry point, and we will need some way to detect
>>> that we are running in PVH mode.
>>>
>>> What issues do you see when using the HVM boot path for kexec?
>> The immediate issue I ran into was ballooning driver over-allocating
>> with XENMEM_populate_physmap:
>>
>> (XEN) Dom15 callback via changed to Direct Vector 0xf3
>> (XEN) d15v0 Over-allocation for domain 15: 262401 > 262400
>> (XEN) memory.c:225:d15v0 Could not allocate order=0 extent: id=15 memflags=0 (175 of 512)
>> (XEN) d15v0 Over-allocation for domain 15: 262401 > 262400
>> (XEN) memory.c:225:d15v0 Could not allocate order=0 extent: id=15 memflags=0 (0 of 512)
>> (XEN) d15v0 Over-allocation for domain 15: 262401 > 262400
>> ...
>>
>> I didn't investigate why it happens, setting xen_pvh=1 helped. Not sure
>> if it's related, but I see the following code in __gnttab_init():
>>
>> 	/* Delay grant-table initialization in the PV on HVM case */
>> 	if (xen_hvm_domain() && !xen_pvh_domain())
>> 		return 0;
>>
>> and gnttab_init() is later called in platform_pci_probe().
> Two more things:
>
> There is xen_pvh_gnttab_setup() initcall which does 
>
>  	if (!xen_pvh_domain())
>                 return -ENODEV;
>
> and this is probably the source of over-allocation.
>
> xen_has_pv_devices() has the following:
>
>         if (xen_pv_domain() || xen_pvh_domain())
>                 return true;
>
> which also has to be patched for PVH-after-kexec. So it seems we either
> have to remove all this stuff somehow or make PVH-ness detectable...
>


Can we read xenstore's dm-version? Or is it too early to get access there?

-boris

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