On 05.03.2019 12:03, Cornelia Huck wrote:
> On Tue, 5 Mar 2019 05:30:01 -0500
> Christian Borntraeger <borntraeger@xxxxxxxxxx> wrote:
>
>> There are cases where halt polling is unwanted. For example when running
>> KVM on an over committed LPAR we rather want to give back the CPU to
>> neighbour LPARs instead of polling. Let us provide a callback that
>> allows architectures to disable polling.
>>
>> Signed-off-by: Christian Borntraeger <borntraeger@xxxxxxxxxx>
>> ---
>> include/linux/kvm_host.h | 10 ++++++++++
>> virt/kvm/Kconfig | 3 +++
>> virt/kvm/kvm_main.c | 2 +-
>> 3 files changed, 14 insertions(+), 1 deletion(-)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index c38cc5eb7e73..5d2bbcf0b1de 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -1283,6 +1283,16 @@ static inline bool vcpu_valid_wakeup(struct kvm_vcpu *vcpu)
>> }
>> #endif /* CONFIG_HAVE_KVM_INVALID_WAKEUPS */
>>
>> +#ifdef CONFIG_HAVE_KVM_NO_POLL
>> +/* Callback that tells if we must not poll */
>> +bool kvm_arch_no_poll(void);
>> +#else
>> +static inline bool kvm_arch_no_poll(void)
>> +{
>> + return false;
>> +}
>> +#endif /* CONFIG_HAVE_KVM_NO_POLL */
>> +
>> #ifdef CONFIG_HAVE_KVM_VCPU_ASYNC_IOCTL
>> long kvm_arch_vcpu_async_ioctl(struct file *filp,
>> unsigned int ioctl, unsigned long arg);
>> diff --git a/virt/kvm/Kconfig b/virt/kvm/Kconfig
>> index ea434ddc8499..aad9284c043a 100644
>> --- a/virt/kvm/Kconfig
>> +++ b/virt/kvm/Kconfig
>> @@ -57,3 +57,6 @@ config HAVE_KVM_VCPU_ASYNC_IOCTL
>>
>> config HAVE_KVM_VCPU_RUN_PID_CHANGE
>> bool
>> +
>> +config HAVE_KVM_NO_POLL
>> + bool
>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>> index 076bc38963bf..c76d83532e14 100644
>> --- a/virt/kvm/kvm_main.c
>> +++ b/virt/kvm/kvm_main.c
>> @@ -2248,7 +2248,7 @@ void kvm_vcpu_block(struct kvm_vcpu *vcpu)
>> u64 block_ns;
>>
>> start = cur = ktime_get();
>> - if (vcpu->halt_poll_ns) {
>> + if (vcpu->halt_poll_ns && !kvm_arch_no_poll()) {
>> ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
>>
>> ++vcpu->stat.halt_attempted_poll;
>
> What this gives us is a way to check whether we want to disable halt
> polling right now (e.g. because we have a high steal time right now).
> Both the drawback and the point in favour is that we check this every
> time we enter kvm_vcpu_block().
>
> Are there (known) scenarios where we might want to disable halt polling
> for some period of time? E.g., when we know via some event that it will
> be counterproductive for some time or until we get an event that tells
> us it might make sense again?
For these cases we probably want to have a userspace tool that sets/unsets
/sys/module/kvm/parameters/halt_poll_ns?
