Re: [PATCH v2 3/4] x86/kvm/hyper-v: direct mode for synthetic timers

[Vitaly Kuznetsov]

Roman Kagan <rkagan@xxxxxxxxxxxxx> writes:

> On Mon, Nov 26, 2018 at 04:47:31PM +0100, Vitaly Kuznetsov wrote:
>> Turns out Hyper-V on KVM (as of 2016) will only use synthetic timers
>> if direct mode is available. With direct mode we notify the guest by
>> asserting APIC irq instead of sending a SynIC message.
>> 
>> The implementation uses existing vec_bitmap for letting lapic code
>> know that we're interested in the particular IRQ's EOI request. We assume
>> that the same APIC irq won't be used by the guest for both direct mode
>> stimer and as sint source (especially with AutoEOI semantics). It is
>> unclear how things should be handled if that's not true.
>> 
>> Direct mode is also somewhat less expensive; in my testing
>> stimer_send_msg() takes not less than 1500 cpu cycles and
>> stimer_notify_direct() can usually be done in 300-400. WS2016 without
>> Hyper-V, however, always sticks to non-direct version.
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>> ---
>> - Changes since v1: avoid open-coding stimer_mark_pending() in
>>   kvm_hv_synic_send_eoi() [Paolo Bonzini]
>> ---
>>  arch/x86/kvm/hyperv.c    | 67 +++++++++++++++++++++++++++++++++++-----
>>  arch/x86/kvm/trace.h     | 10 +++---
>>  arch/x86/kvm/x86.c       |  1 +
>>  include/uapi/linux/kvm.h |  1 +
>>  4 files changed, 67 insertions(+), 12 deletions(-)
>> 
>> diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c
>> index eaec15c738df..9533133be566 100644
>> --- a/arch/x86/kvm/hyperv.c
>> +++ b/arch/x86/kvm/hyperv.c
>> @@ -38,6 +38,9 @@
>>  
>>  #define KVM_HV_MAX_SPARSE_VCPU_SET_BITS DIV_ROUND_UP(KVM_MAX_VCPUS, 64)
>>  
>> +static void stimer_mark_pending(struct kvm_vcpu_hv_stimer *stimer,
>> +				bool vcpu_kick);
>> +
>>  static inline u64 synic_read_sint(struct kvm_vcpu_hv_synic *synic, int sint)
>>  {
>>  	return atomic64_read(&synic->sint[sint]);
>> @@ -53,8 +56,21 @@ static inline int synic_get_sint_vector(u64 sint_value)
>>  static bool synic_has_vector_connected(struct kvm_vcpu_hv_synic *synic,
>>  				      int vector)
>>  {
>> +	struct kvm_vcpu *vcpu = synic_to_vcpu(synic);
>> +	struct kvm_vcpu_hv *hv_vcpu = vcpu_to_hv_vcpu(vcpu);
>> +	struct kvm_vcpu_hv_stimer *stimer;
>>  	int i;
>>  
>> +	for (i = 0; i < ARRAY_SIZE(hv_vcpu->stimer); i++) {
>> +		stimer = &hv_vcpu->stimer[i];
>> +		if (stimer->config.enable && stimer->config.direct_mode &&
>> +		    stimer->config.apic_vector == vector)
>> +			return true;
>> +	}
>> +
>> +	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
>> +		return false;
>> +
>>  	for (i = 0; i < ARRAY_SIZE(synic->sint); i++) {
>>  		if (synic_get_sint_vector(synic_read_sint(synic, i)) == vector)
>>  			return true;
>> @@ -80,14 +96,14 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic,
>>  static void synic_update_vector(struct kvm_vcpu_hv_synic *synic,
>>  				int vector)
>>  {
>> -	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
>> -		return;
>> -
>>  	if (synic_has_vector_connected(synic, vector))
>>  		__set_bit(vector, synic->vec_bitmap);
>>  	else
>>  		__clear_bit(vector, synic->vec_bitmap);
>>  
>> +	if (vector < HV_SYNIC_FIRST_VALID_VECTOR)
>> +		return;
>> +
>
> Just noticed that the patch seems to assume that "direct" timers are
> allowed to use any vectors including 0-15.  I guess this is incorrect,
> and instead stimer_set_config should error out on direct mode with a
> vector less than HV_SYNIC_FIRST_VALID_VECTOR.

The spec is really vague about this and I'm not sure that this has
anything to do with HV_SYNIC_FIRST_VALID_VECTOR (as these are actually
not "synic" vectors, I *think* that SynIC doesn't even need to be
enabled to make them work).

I checked and Hyper-V 2016 uses vector '0xff', not sure if it proves
your point :-)

Do you envision any issues in KVM if we keep allowing vectors <
HV_SYNIC_FIRST_VALID_VECTOR?

-- 
Vitaly