Re: [PATCH] ppc/spapr: Advertise StoreEOI for POWER10 compat guests

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 2/17/22 10:23, Cédric Le Goater wrote:
> On 2/17/22 12:28, Daniel Henrique Barboza wrote:
>>
>>
>> On 2/14/22 11:11, Cédric Le Goater wrote:
>>> When an interrupt has been handled, the OS notifies the interrupt
>>> controller with a EOI sequence. On a POWER9 and POWER10 systems using
>>> the XIVE interrupt controller, this can be done with a load or a store
>>> operation on the ESB interrupt management page of the interrupt. The
>>> StoreEOI operation has less latency and improves interrupt handling
>>> performance but it was deactivated during the POWER9 DD2.0 timeframe
>>> because of ordering issues. POWER9 systems use the LoadEOI instead.
>>> POWER10 compat guests should have fixed the issue with
>>> Load-after-Store ordering and StoreEOI can be activated for them
>>> again.
>>>
>>> To maintain performance, this ordering is only enforced for the
>>> XIVE_ESB_SET_PQ_10 load operation. This operation can be used to
>>> disable temporarily an interrupt source. If StoreEOI is active, a
>>> source could be left enabled if the load and store operations come
>>> out of order.
>>>
>>> Add a check in our XIVE emulation model for Load-after-Store when
>>> StoreEOI is active. It should catch unreliable sequences. Other load
>>> operations should be fine without it.
>>>
>>> Signed-off-by: Cédric Le Goater <clg@kaod.org>
>>> ---
>>
>> Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
> 
> 
> Unfortunetaly, this patch breaks migration under TCG because the XIVE
> source flag is not updated on the target side. KVM is not impacted
> because the emulated sources are not used. This needs to be addressed
> in a v2.
> 
> That said, even without this patch, TCG migration is broken. some CPUs
> on the receive side are stalled on CPU Hard LOCKUPs. QEMU 6.2 is impacted.
> So it has been a while :/



I've done a few tests and I can see Hard Lockups with TCG pseries migration, when using
multiples CPUs (I used -smp 4 like you suggested in private), since at least QEMU
v6.0.0.

This is hardly surprising since TCG migration isn't something that we ever supported in
a product or even in the community*. It would be good to understand why and get it fixed,
but for now we can take a bit comfort in knowing that:

- it has been broken for awhile (if ever worked). If this was a recent 7.0 regression
we would need to solve it for this upcoming release;

- single CPU TCG migration seems to be working fine, so we can count with this TCG
migration scenario for testing.



* I'm hoping David and Greg can push back on this if my assumption is wrong.



Thanks,



Daniel






> 
> See below.
> 
> C.
> 
> 
> 
> [   24.113608] watchdog: CPU 0 detected hard LOCKUP on other CPUs 1,3
> [   24.116534] watchdog: CPU 0 TB:15585461459, last SMP heartbeat TB:7394335409 (15998ms ago)
> [   24.117840] watchdog: CPU 1 Hard LOCKUP
> [   24.117956] watchdog: CPU 1 TB:15587843000, last heartbeat TB:5355690415 (19984ms ago)
> [   24.117999] Modules linked in:
> [   24.118387] irq event stamp: 341399
> [   24.118399] hardirqs last  enabled at (341399): [<c000000000caea2c>] snooze_loop+0x9c/0x290
> [   24.118900] hardirqs last disabled at (341398): [<c000000000208b9c>] do_idle+0x12c/0x450
> [   24.118943] softirqs last  enabled at (9798): [<c000000000f97dfc>] __do_softirq+0x60c/0x678
> [   24.118971] softirqs last disabled at (9789): [<c0000000001b06f8>] __irq_exit_rcu+0x158/0x1c0
> [   24.119127] CPU: 1 PID: 0 Comm: swapper/1 Not tainted 5.17.0-rc4-dirty #984
> [   24.119293] NIP:  c000000000caea78 LR: c000000000caea38 CTR: c000000000cae990
> [   24.119315] REGS: c0000000fff43d60 TRAP: 0100   Not tainted  (5.17.0-rc4-dirty)
> [   24.119352] MSR:  800000000280b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE>  CR: 28000228  XER: 00000006
> [   24.119554] CFAR: c000000000caea98 IRQMASK: 0
> [   24.119554] GPR00: c000000000caea2c c000000002bbbd80 c000000001c30b00 0000000000000000
> [   24.119554] GPR04: 0000000000000006 0000000000000000 000000000000c800 c000000001c7dc38
> [   24.119554] GPR08: c000000002b5d500 0000000000000000 00000003a115ef39 36d551ed00000000
> [   24.119554] GPR12: c000000000cae990 c0000000fffff300 0000000000000000 0000000000000000
> [   24.119554] GPR16: 0000000000000000 0000000000000000 0000000000000000 0000000000000000
> [   24.119554] GPR20: 0000000000000000 0000000000000000 0000000000000000 c000000001b3a660
> [   24.119554] GPR24: c0000000ffa4fb48 000000059d7c5070 c000000001c78e48 0000000000000000
> [   24.119554] GPR28: c000000001b3a660 c0000000015422e0 c0000000015422e8 0000000000000000
> [   24.119845] NIP [c000000000caea78] snooze_loop+0xe8/0x290
> [   24.119866] LR [c000000000caea38] snooze_loop+0xa8/0x290
> [   24.119998] Call Trace:
> [   24.120029] [c000000002bbbd80] [c000000000caea2c] snooze_loop+0x9c/0x290 (unreliable)
> [   24.120097] [c000000002bbbdc0] [c000000000cab730] cpuidle_enter_state+0x300/0x730
> [   24.120119] [c000000002bbbe30] [c000000000cabbfc] cpuidle_enter+0x4c/0x70
> [   24.120131] [c000000002bbbe70] [c000000000208d98] do_idle+0x328/0x450
> [   24.120141] [c000000002bbbf00] [c00000000020926c] cpu_startup_entry+0x3c/0x40
> [   24.120150] [c000000002bbbf30] [c00000000005e144] start_secondary+0x2a4/0x2b0
> [   24.120161] [c000000002bbbf90] [c00000000000d054] start_secondary_prolog+0x10/0x14
> [   24.120238] Instruction dump:
> [   24.120320] e9280080 e8c7d148 3ce20005 71290004 38e7d138 7d4a3214 4082003c 60000000
> [   24.120357] 60000000 60420000 7c210b78 7ffffb78 <8927000c> 2c090000 41820010 7d2c42a6
>