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

[Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>]

Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> writes:

> 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().

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...

-- 
  Vitaly

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