On 9/23/22 3:45 AM, Borislav Petkov wrote:
> + kvm ML and leaving the whole mail quoted in for them.
>
> On Fri, Sep 23, 2022 at 09:05:26AM +0200, Peter Zijlstra wrote:
>> On Thu, Jul 21, 2022 at 01:44:33PM -0700, Srivatsa S. Bhat wrote:
>>> From: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>
>>>
>>> VMware ESXi allows enabling a passthru mwait CPU-idle state in the
>>> guest using the following VMX option:
>>>
>>> monitor_control.mwait_in_guest = "TRUE"
>>>
>>> This lets a vCPU in mwait to remain in guest context (instead of
>>> yielding to the hypervisor via a VMEXIT), which helps speed up
>>> wakeups from idle.
>>>
>>> However, this runs into problems with CPU hotplug, because the Linux
>>> CPU offline path prefers to put the vCPU-to-be-offlined in mwait
>>> state, whenever mwait is available. As a result, since a vCPU in mwait
>>> remains in guest context and does not yield to the hypervisor, an
>>> offline vCPU *appears* to be 100% busy as viewed from ESXi, which
>>> prevents the hypervisor from running other vCPUs or workloads on the
>>> corresponding pCPU (particularly when vCPU - pCPU mappings are
>>> statically defined by the user).
>>
>> I would hope vCPU pinning is a mandatory thing when MWAIT passthrough it
>> set?
>>
>>> [ Note that such a vCPU is not
>>> actually busy spinning though; it remains in mwait idle state in the
>>> guest ].
>>>
>>> Fix this by overriding the CPU offline play_dead() callback for VMware
>>> hypervisor, by putting the CPU in halt state (which actually yields to
>>> the hypervisor), even if mwait support is available.
>>>
>>> Signed-off-by: Srivatsa S. Bhat (VMware) <srivatsa@xxxxxxxxxxxxx>
>>> ---
>>
>>> +static void vmware_play_dead(void)
>>> +{
>>> + play_dead_common();
>>> + tboot_shutdown(TB_SHUTDOWN_WFS);
>>> +
>>> + /*
>>> + * Put the vCPU going offline in halt instead of mwait (even
>>> + * if mwait support is available), to make sure that the
>>> + * offline vCPU yields to the hypervisor (which may not happen
>>> + * with mwait, for example, if the guest's VMX is configured
>>> + * to retain the vCPU in guest context upon mwait).
>>> + */
>>> + hlt_play_dead();
>>> +}
>>> #endif
>>>
>>> static __init int activate_jump_labels(void)
>>> @@ -349,6 +365,7 @@ static void __init vmware_paravirt_ops_setup(void)
>>> #ifdef CONFIG_SMP
>>> smp_ops.smp_prepare_boot_cpu =
>>> vmware_smp_prepare_boot_cpu;
>>> + smp_ops.play_dead = vmware_play_dead;
>>> if (cpuhp_setup_state_nocalls(CPUHP_AP_ONLINE_DYN,
>>> "x86/vmware:online",
>>> vmware_cpu_online,
>>
>> No real objection here; but would not something like the below fix the
>> problem more generally? I'm thinking MWAIT passthrough for *any*
>> hypervisor doesn't want play_dead to use it.
>>
That would be better indeed, thank you!
>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index f24227bc3220..166cb3aaca8a 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -1759,6 +1759,8 @@ static inline void mwait_play_dead(void)
>> return;
>> if (!this_cpu_has(X86_FEATURE_CLFLUSH))
>> return;
>> + if (this_cpu_has(X86_FEATURE_HYPERVISOR))
>> + return;
>> if (__this_cpu_read(cpu_info.cpuid_level) < CPUID_MWAIT_LEAF)
>> return;
>
> Yeah, it would be nice if we could get a consensus here from all
> relevant HVs.
>
I'll send out a v2 after trying out these changes.
Thank you!
Regards,
Srivatsa
VMware Photon OS
