* [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU
@ 2022-11-14 16:01 Laurent Dufour
2022-11-18 17:52 ` Nathan Lynch
2022-12-08 12:40 ` Michael Ellerman
0 siblings, 2 replies; 3+ messages in thread
From: Laurent Dufour @ 2022-11-14 16:01 UTC (permalink / raw)
To: linuxppc-dev; +Cc: Nathan Lynch, npiggin
The VPA should unregister when offlining a CPU. Otherwise there could be a
short window where 2 CPUs could share the same VPA.
This happens because the hypervisor is still keeping the VPA attached to
the vCPU even if it became offline.
Here is a potential situation:
1. remove proc A,
2. add proc B. If proc B gets proc A's place in cpu_present_map, then it
registers proc A's VPAs.
3. If proc B is then re-added to the LP, its threads are sharing VPAs with
proc A briefly as they come online.
As the hypervisor may check for the VPA's yield_count field oddity, it may
detects an unexpected value and kill the LPAR.
Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
---
arch/powerpc/platforms/pseries/hotplug-cpu.c | 1 +
1 file changed, 1 insertion(+)
diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
index e0a7ac5db15d..090ae5a1e0f5 100644
--- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
+++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
@@ -70,6 +70,7 @@ static void pseries_cpu_offline_self(void)
xics_teardown_cpu();
unregister_slb_shadow(hwcpu);
+ unregister_vpa(hwcpu);
rtas_stop_self();
/* Should never get here... */
--
2.38.1
^ permalink raw reply related [flat|nested] 3+ messages in thread
* Re: [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU
2022-11-14 16:01 [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU Laurent Dufour
@ 2022-11-18 17:52 ` Nathan Lynch
2022-12-08 12:40 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Nathan Lynch @ 2022-11-18 17:52 UTC (permalink / raw)
To: Laurent Dufour; +Cc: linuxppc-dev, npiggin
Laurent Dufour <ldufour@linux.ibm.com> writes:
> The VPA should unregister when offlining a CPU. Otherwise there could be a
> short window where 2 CPUs could share the same VPA.
>
> This happens because the hypervisor is still keeping the VPA attached to
> the vCPU even if it became offline.
>
> Here is a potential situation:
> 1. remove proc A,
> 2. add proc B. If proc B gets proc A's place in cpu_present_map, then it
> registers proc A's VPAs.
> 3. If proc B is then re-added to the LP, its threads are sharing VPAs with
> proc A briefly as they come online.
>
> As the hypervisor may check for the VPA's yield_count field oddity, it may
> detects an unexpected value and kill the LPAR.
>
> Suggested-by: Nathan Lynch <nathanl@linux.ibm.com>
> Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
> ---
> arch/powerpc/platforms/pseries/hotplug-cpu.c | 1 +
> 1 file changed, 1 insertion(+)
>
> diff --git a/arch/powerpc/platforms/pseries/hotplug-cpu.c b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> index e0a7ac5db15d..090ae5a1e0f5 100644
> --- a/arch/powerpc/platforms/pseries/hotplug-cpu.c
> +++ b/arch/powerpc/platforms/pseries/hotplug-cpu.c
> @@ -70,6 +70,7 @@ static void pseries_cpu_offline_self(void)
> xics_teardown_cpu();
>
> unregister_slb_shadow(hwcpu);
> + unregister_vpa(hwcpu);
> rtas_stop_self();
>
> /* Should never get here... */
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
I was wondering whether we could leave an active dispatch trace log
buffer registered, which could interfere with releasing the VPA, but I
verified that DTL has the appropriate cpuhp callback for that
(dtl_worker_offline()).
Alternatively we could change the code to dynamically register and
unregister VPAs only on processor add and remove, as opposed to CPU
online/offline. But I can't see any significant advantage to that.
^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU
2022-11-14 16:01 [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU Laurent Dufour
2022-11-18 17:52 ` Nathan Lynch
@ 2022-12-08 12:40 ` Michael Ellerman
1 sibling, 0 replies; 3+ messages in thread
From: Michael Ellerman @ 2022-12-08 12:40 UTC (permalink / raw)
To: linuxppc-dev, Laurent Dufour; +Cc: Nathan Lynch, npiggin
On Mon, 14 Nov 2022 17:01:50 +0100, Laurent Dufour wrote:
> The VPA should unregister when offlining a CPU. Otherwise there could be a
> short window where 2 CPUs could share the same VPA.
>
> This happens because the hypervisor is still keeping the VPA attached to
> the vCPU even if it became offline.
>
> Here is a potential situation:
> 1. remove proc A,
> 2. add proc B. If proc B gets proc A's place in cpu_present_map, then it
> registers proc A's VPAs.
> 3. If proc B is then re-added to the LP, its threads are sharing VPAs with
> proc A briefly as they come online.
>
> [...]
Applied to powerpc/next.
[1/1] powerpc/pseries: unregister VPA when hot unplugging a CPU
https://git.kernel.org/powerpc/c/f6aa37c51ec0d053ee34c235bfe0e666618a3baf
cheers
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2022-12-08 12:55 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2022-11-14 16:01 [PATCH] powerpc/pseries: unregister VPA when hot unplugging a CPU Laurent Dufour
2022-11-18 17:52 ` Nathan Lynch
2022-12-08 12:40 ` Michael Ellerman
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).