[Xen-devel] kexec -e in PVHVM guests (and in PV).

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

Hey, 

I had on my todo list an patch from Olaf patch that shuffles
the shared_page to be in the 0xFE700000 addr (in the "gap"
with newer QEMU's) which unfortunately did not work when
migrating on 32-bit PVHVM guests on Xen 4.1.

The commit is 9d02b43dee0d7fb18dfb13a00915550b1a3daa9f
"xen PVonHVM: use E820_Reserved area for shared_info" and it
ended up being reverted. I dusted it off and I think I found
the original bug (and fixed it), but while digging in this
the more I discovered a ton more of issues.

A bit about the use case - the 'kexec -e' allows one to
restart the Linux kernel without a reboot. It is not a crash kernel
so it is just meant to restart and work, and then restart, etc.

The 'kdump -c' (crash) is a different use case and I had not
thought much about it. But I think that all of the solutions
I am thinking of will make it also work. (so you could
do kexec-crash -> kexec-e->kexec-e>kexec-crash->kexec-e, and
so, if you would want to).

The problem I uncovered was that the memory region where
the new kernel would be executed had bits of memory changed - which
meant that the purgatory code in kexec would detect the SHA1SUM
being incorrect and not load. That lead me to find out that
VCPUOP_register_vcpu_info was the culprit (well, the xen_vcpu_info
was being modified, and its PFN was in the 'new' kernel image area).

Anyhow, the end result of that is that I think to get this
working we would need to have:

 1). A symmetrical VCPUOP_register_vcpu_info call, say
     VCPUOP_unregister_vcpu_info, which would for a provided vpuid
     set 'vcpu_info' to the shared_info, and 'vcpu_info_mfn' to
     INVALID_MFN. Naturally the vcpu_id has to be down (_VPF_down).
     A prototype patch along with an naive implementation in
     the Linux kernel made this work surprisingly well!

     The Linux kernel had to call on the shutdown the:
     disable_nonboot_cpus() which would bring all the AP CPUs down.
     Each AP CPU would call said hypercall. Also on each CPU
     bringup we would call this (that is the BSP would make this
     call before bringing the AP CPUs up - on bootup paths it
     would result in nothing, while for an kexec -c type kernel
     it would allow us to use the CPUs).

 2). Ditto for VCPUOP_register_runtime and
     VCPUOP_register_runstate_memory_area.  They would need a
     similar 'unregister' call with similar semantics as the
     one above.

 3). The shared_info. Olaf's patch stuck the shared_info in the
     "gaps" of the E820 or the E820_RSRV region. But the recent patches
      for PCI passthrough are making me twitchy and I think we would
      need to parse the E820 and /proc/ioports (so 'resource API in
      Linux kernel' to figure out a good place to stash this. Or on
      shutdown (kexec -e)  balloon out the shared region (need to
      double check that this possible in the first place).

 4). Balloon memory. I am not really sure how to deal with that. The
     guest might have ballooned out tons of memory but the new kernel
     won't know about it until the xen/balloon driver kicks in and
     figures this out based on XenStore. Then it will try to balloon
     out.. and depending on its luck - balloon out memory that was
     already ballooned out, or not.  Also during the bootup of
     the 'kexec -e' kernel it might touch pages that had been
     ballooned out - and try to use them!
 
 5). Events. Olaf had written code long time ago that would poke the
     events to see if they were already in use (-EEXIST) and if so
     re-use them - it works great albeit there are tons of messages
     in the Xen ring buffer. The Linux patch I wrote did an
     'disable_nonboot_cpus' and also tore down the BSP interrupts - that
     meant that all of the events were nicely torn down. This all works
     on non-FIFO event.  David Vrabel says that to make this work
     (re-use or teardown and bring up) would be hard.
   
 6). QEMU PnP typ devices. Such as 'serial,'i8042', and 'rtc' end up
     going through the EVTCHNOP_bind_pirg. Somehow on the 'kexec -e'
     kernel we end up doing OK, but the devices don't work anymore.
     That is - the serial input does not accept any more input (but
     it can output alright).

 7). Grants. Andrew Cooper hinted at this and a bit of experimentation
     shows that Xen hypervisor will indeed smack down any guest that
     tries to re-use its "old" grants. I am not even sure if the
     GNTTAB_setup call is returning the "old" grant frames.
     His suggestion was 'GNTTAB_reset' to well, reset everything.

My thinking is that a lot of this code is shared with PV (and PVH)
once this is fixed we could do full scale 'kexec -e' in an PV
(or PVH) type guest. Doing dom0 kexec -e would be an interesting
experiment :-(

I am unable to fix this for Xen 4.5 and I am not sure what other
issues there are present. If folks have some ideas or would like to
chime in (or even pick some of these up!)- please do respond.

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