Re: [Xen-devel] [BUG] XEN domU crash when PV grub chainloads 32-bit domU grub

[Vladimir 'Ï-coder/phcoder' Serbinenko <phcoder@xxxxxxxxx>]

On 22.09.2015 10:53, Ian Campbell wrote:
> Hi Vladimir & grub-devel,
> 
> Do you have any thoughts on this issue with i386 pv-grub2?
> 
Is it still an issue? If so I'll try to replicate it. From stack dump I
see that it has jumped to NULL. GRUB has no threads so it's not a race
condition with itself but may be one with some Xen part. An altrnative
possibility is that grub forgets to flush cache at some point in boot
process.
> Thanks, Ian.
> 
> On Mon, 2015-09-21 at 22:03 +0200, Andreas Sundstrom wrote:
>> This is using Debian Jessie and grub 2.02~beta2-22 (with Debian patches
>> applied) and Xen 4.4.1
>>
>> I originally posted a bug report with Debian but got the suggestion to
>> file bugs with upstream as well.
>> Debian bug report:
>> https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=799480
>>
>> Note that my original thought was that this bug probably is within GRUB.
>> But Ian asked me to file a bug with Xen as well, you have to live with
>> the
>> fact that it is centered around GRUB though.
>>
>> Here's the information from my original bug report:
>>
>> Using 64-bit dom0 and 32-bit domU PV (para-virtualized) grub sometimes
>> fail when chainloading the domU's grub. 64-bit domU seem to work 100%
>> of the time.
>>
>> My understanding of the process:
>>
>>  * dom0 launches domU with grub that is loaded from dom0's disk.
>>  * Grub reads config file from memdisk, and then looks for grub binary in
>>     domU filesystem.
>>  * If grub is found in domU it then chainloads (multiboot) that grub
>> binary
>>     and the domU grub reads grub.cfg and continue booting.
>>  * If grub is not found in domU it reads grub.cfg and continues with
>> boot.
>>
>> It fails at step 3 in my list of the boot process, but sometimes it
>> does work so it may be something like a race condition that causes the
>> problem?
>>
>> A workaround is to not install or rename /boot/xen in domU so that the
>> first grub that is loaded from dom0's disk will not find the grub
>> binary in the domU filesystem and hence continues to read grub.cfg and
>> boot. The drawback of this is of course that the two versions can't
>> differ too much as there are different setups creating grub.cfg and
>> then reading/parsing it at boot time.
>>
>> I am not sure at this point whether this is a problem in XEN or a
>> problem in grub but I compiled the legacy pvgrub that uses some minios
>> from XEN (don't really know much more about it) and when that legacy
>> pvgrub chainloads the domU grub it seems to work 100% of the time. Now
>> the legace pvgrub is not a real alternative as it's not packaged for
>> Debian though.
>>
>> When it fails "xl create vm -c" outputs this:
>> Parsing config from /etc/xen/vm
>> libxl: error: libxl_dom.c:35:libxl__domain_type: unable to get domain
>> type for domid=16
>> Unable to attach console
>> libxl: error: libxl_exec.c:118:libxl_report_child_exitstatus: console
>> child [0] exited with error status 1
>>
>> And "xl dmesg" shows errors like this:
>> (XEN) traps.c:2514:d15 Domain attempted WRMSR 00000000c0010201 from
>> 0x0000000000000000 to 0x000000000000ffff.
>> (XEN) d16:v0: unhandled page fault (ec=0010)
>> (XEN) Pagetable walk from 0000000000000000:
>> (XEN) L4[0x000] = 0000000200256027 000000000000049c
>> (XEN) L3[0x000] = 0000000200255027 000000000000049d
>> (XEN) L2[0x000] = 0000000200251023 00000000000004a1
>> (XEN) L1[0x000] = 0000000000000000 ffffffffffffffff
>> (XEN) domain_crash_sync called from entry.S: fault at ffff82d08021feb0
>> compat_create_bounce_frame+0xc6/0xde
>> (XEN) Domain 16 (vcpu#0) crashed on cpu#0:
>> (XEN) ----[ Xen-4.4.1 x86_64 debug=n Not tainted ]----
>> (XEN) CPU: 0
>> (XEN) RIP: e019:[<0000000000000000>]
>> (XEN) RFLAGS: 0000000000000246 EM: 1 CONTEXT: pv guest
>> (XEN) rax: 0000000000000000 rbx: 0000000000000000 rcx: 0000000000000000
>> (XEN) rdx: 0000000000000000 rsi: 0000000000499000 rdi: 0000000000800000
>> (XEN) rbp: 000000000000000a rsp: 00000000005a5ff0 r8: 0000000000000000
>> (XEN) r9: 0000000000000000 r10: ffff83023e9b9000 r11: ffff83023e9b9000
>> (XEN) r12: 0000033f3d335bfb r13: ffff82d080300800 r14: ffff82d0802ea940
>> (XEN) r15: ffff83005e819000 cr0: 000000008005003b cr4: 00000000000506f0
>> (XEN) cr3: 0000000200b7a000 cr2: 0000000000000000
>> (XEN) ds: e021 es: e021 fs: e021 gs: e021 ss: e021 cs: e019
>> (XEN) Guest stack trace from esp=005a5ff0:
>> (XEN) 00000010 00000000 0001e019 00010046 0016b38b 0016b38a 0016b389
>> 0016b388
>> (XEN) 0016b387 0016b386 0016b385 0016b384 0016b383 0016b382 0016b381
>> 0016b380
>> (XEN) 0016b37f 0016b37e 0016b37d 0016b37c 0016b37b 0016b37a 0016b379
>> 0016b378
>> (XEN) 0016b377 0016b376 0016b375 0016b374 0016b373 0016b372 0016b371
>> 0016b370
>> (XEN) 0016b36f 0016b36e 0016b36d 0016b36c 0016b36b 0016b36a 0016b369
>> 0016b368
>> (XEN) 0016b367 0016b366 0016b365 0016b364 0016b363 0016b362 0016b361
>> 0016b360
>> (XEN) 0016b35f 0016b35e 0016b35d 0016b35c 0016b35b 0016b35a 0016b359
>> 0016b358
>> (XEN) 0016b357 0016b356 0016b355 0016b354 0016b353 0016b352 0016b351
>> 0016b350
>> (XEN) 0016b34f 0016b34e 0016b34d 0016b34c 0016b34b 0016b34a 0016b349
>> 0016b348
>> (XEN) 0016b347 0016b346 0016b345 0016b344 0016b343 0016b342 0016b341
>> 0016b340
>> (XEN) 0016b33f 0016b33e 0016b33d 0016b33c 0016b33b 0016b33a 0016b339
>> 0016b338
>> (XEN) 0016b337 0016b336 0016b335 0016b334 0016b333 0016b332 0016b331
>> 0016b330
>> (XEN) 0016b32f 0016b32e 0016b32d 0016b32c 0016b32b 0016b32a 0016b329
>> 0016b328
>> (XEN) 0016b327 0016b326 0016b325 0016b324 0016b323 0016b322 0016b321
>> 0016b320
>> (XEN) 0016b31f 0016b31e 0016b31d 0016b31c 0016b31b 0016b31a 0016b319
>> 0016b318
>> (XEN) 0016b317 0016b316 0016b315 0016b314 0016b313 0016b312 0016b311
>> 0016b310
>> (XEN) 0016b30f 0016b30e 0016b30d 0016b30c 0016b30b 0016b30a 0016b309
>> 0016b308
>> (XEN) 0016b307 0016b306 0016b305 0016b304 0016b303 0016b302 0016b301
>> 0016b300
>> (XEN) 0016b2ff 0016b2fe 0016b2fd 0016b2fc 0016b2fb 0016b2fa 0016b2f9
>> 0016b2f8
>> (XEN) 0016b2f7 0016b2f6 0016b2f5 0016b2f4 0016b2f3 0016b2f2 0016b2f1
>> 0016b2f0
>>
>> An easy way to find out which grub you are in if the machine boots is
>> to hit 'c' and type 'ls', only the grub from dom0 will know about
>> (memdisk). So when trying to replicate the issue (and the domU
>> actually starts) you can hit 'c', type 'ls' (check for memdisk) and
>> then type 'halt' and relaunch the domU. Usually I can't launch more
>> than 4-5 times in a row before it fails, often it fails on my first
>> try.
>>
>> For information I have reproduced on two different AMD desktop
>> processor machines, not sure if Intel would be any different. I'm
>> pretty sure I did tests with grub from unstable with same result at
>> some point, but can test again if that is likely to work.
>>
>> The package that is in installed on the domU side is "grub-xen".
>>
>> I am unable to understand how to debug grub further on my own, I have
>> printed out text from grub so that I understood that it is the
>> chainload that fails. I see no output from the domU grub (except when
>> it works as it should of course). I can help with further testing if
>> needed.
>>
>> /Andreas
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@xxxxxxxxxxxxx
>> http://lists.xen.org/xen-devel
>

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