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

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

Roger Pau Monne <roger.pau@xxxxxxxxxx> writes:

> On Tue, Mar 21, 2017 at 10:21:52AM +0100, Vitaly Kuznetsov wrote:
>> Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx> 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().

-- 
  Vitaly

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