Re: [PATCH 3/7] KVM-HV: KVM Steal time implementation

Glauber Costa
2011-06-16T12:11:42Z

On 06/16/2011 08:27 AM, Gleb Natapov wrote:
> On Thu, Jun 16, 2011 at 12:31:02AM -0300, Glauber Costa wrote:
>> On 06/15/2011 06:09 AM, Gleb Natapov wrote:
>>> On Wed, Jun 15, 2011 at 12:09:31AM -0300, Glauber Costa wrote:
>>>> On 06/14/2011 04:45 AM, Gleb Natapov wrote:
>>>>> On Mon, Jun 13, 2011 at 07:31:33PM -0400, Glauber Costa wrote:
>>>>>> To implement steal time, we need the hypervisor to pass the guest information
>>>>>> about how much time was spent running other processes outside the VM.
>>>>>> This is per-vcpu, and using the kvmclock structure for that is an abuse
>>>>>> we decided not to make.
>>>>>>
>>>>>> In this patchset, I am introducing a new msr, KVM_MSR_STEAL_TIME, that
>>>>>> holds the memory area address containing information about steal time
>>>>>>
>>>>>> This patch contains the hypervisor part for it. I am keeping it separate from
>>>>>> the headers to facilitate backports to people who wants to backport the kernel
>>>>>> part but not the hypervisor, or the other way around.
>>>>>>
>>>>>> Signed-off-by: Glauber Costa<glommer@redhat.com>
>>>>>> CC: Rik van Riel<riel@redhat.com>
>>>>>> CC: Jeremy Fitzhardinge<jeremy.fitzhardinge@citrix.com>
>>>>>> CC: Peter Zijlstra<peterz@infradead.org>
>>>>>> CC: Avi Kivity<avi@redhat.com>
>>>>>> CC: Anthony Liguori<aliguori@us.ibm.com>
>>>>>> CC: Eric B Munson<emunson@mgebm.net>
>>>>>> ---
>>>>>> arch/x86/include/asm/kvm_host.h | 8 +++++
>>>>>> arch/x86/include/asm/kvm_para.h | 4 ++
>>>>>> arch/x86/kvm/x86.c | 60 +++++++++++++++++++++++++++++++++++++--
>>>>>> 3 files changed, 69 insertions(+), 3 deletions(-)
>>>>>>
>>>>>> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
>>>>>> index fc38eca..5dce014 100644
>>>>>> --- a/arch/x86/include/asm/kvm_host.h
>>>>>> +++ b/arch/x86/include/asm/kvm_host.h
>>>>>> @@ -388,6 +388,14 @@ struct kvm_vcpu_arch {
>>>>>> unsigned int hw_tsc_khz;
>>>>>> unsigned int time_offset;
>>>>>> struct page *time_page;
>>>>>> +
>>>>>> + struct {
>>>>>> + u64 msr_val;
>>>>>> + gpa_t stime;
>>>>>> + struct kvm_steal_time steal;
>>>>>> + u64 this_time_out;
>>>>>> + } st;
>>>>>> +
>>>>>> u64 last_guest_tsc;
>>>>>> u64 last_kernel_ns;
>>>>>> u64 last_tsc_nsec;
>>>>>> diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
>>>>>> index ac306c4..0341e61 100644
>>>>>> --- a/arch/x86/include/asm/kvm_para.h
>>>>>> +++ b/arch/x86/include/asm/kvm_para.h
>>>>>> @@ -45,6 +45,10 @@ struct kvm_steal_time {
>>>>>> __u32 pad[6];
>>>>>> };
>>>>>>
>>>>>> +#define KVM_STEAL_ALIGNMENT_BITS 5
>>>>>> +#define KVM_STEAL_VALID_BITS ((-1ULL<< (KVM_STEAL_ALIGNMENT_BITS + 1)))
>>>>>> +#define KVM_STEAL_RESERVED_MASK (((1<< KVM_STEAL_ALIGNMENT_BITS) - 1 )<< 1)
>>>>>> +
>>>>>> #define KVM_MAX_MMU_OP_BATCH 32
>>>>>>
>>>>>> #define KVM_ASYNC_PF_ENABLED (1<< 0)
>>>>>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>>>>>> index 6645634..10fe028 100644
>>>>>> --- a/arch/x86/kvm/x86.c
>>>>>> +++ b/arch/x86/kvm/x86.c
>>>>>> @@ -797,12 +797,12 @@ EXPORT_SYMBOL_GPL(kvm_get_dr);
>>>>>> * kvm-specific. Those are put in the beginning of the list.
>>>>>> */
>>>>>>
>>>>>> -#define KVM_SAVE_MSRS_BEGIN 8
>>>>>> +#define KVM_SAVE_MSRS_BEGIN 9
>>>>>> static u32 msrs_to_save[] = {
>>>>>> MSR_KVM_SYSTEM_TIME, MSR_KVM_WALL_CLOCK,
>>>>>> MSR_KVM_SYSTEM_TIME_NEW, MSR_KVM_WALL_CLOCK_NEW,
>>>>>> HV_X64_MSR_GUEST_OS_ID, HV_X64_MSR_HYPERCALL,
>>>>>> - HV_X64_MSR_APIC_ASSIST_PAGE, MSR_KVM_ASYNC_PF_EN,
>>>>>> + HV_X64_MSR_APIC_ASSIST_PAGE, MSR_KVM_ASYNC_PF_EN, MSR_KVM_STEAL_TIME,
>>>>>> MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
>>>>>> MSR_STAR,
>>>>>> #ifdef CONFIG_X86_64
>>>>>> @@ -1480,6 +1480,34 @@ static void kvmclock_reset(struct kvm_vcpu *vcpu)
>>>>>> }
>>>>>> }
>>>>>>
>>>>>> +static void record_steal_time(struct kvm_vcpu *vcpu)
>>>>>> +{
>>>>>> + u64 delta;
>>>>>> +
>>>>>> + if (vcpu->arch.st.stime&& vcpu->arch.st.this_time_out) {
>>>>>> +
>>>>>> + if (unlikely(kvm_read_guest(vcpu->kvm, vcpu->arch.st.stime,
>>>>>> + &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)))) {
>>>>>> +
>>>>>> + vcpu->arch.st.stime = 0;
>>>>>> + return;
>>>>>> + }
>>>>>> +
>>>>>> + delta = (get_kernel_ns() - vcpu->arch.st.this_time_out);
>>>>>> +
>>>>>> + vcpu->arch.st.steal.steal += delta;
>>>>>> + vcpu->arch.st.steal.version += 2;
>>>>>> +
>>>>>> + if (unlikely(kvm_write_guest(vcpu->kvm, vcpu->arch.st.stime,
>>>>> Why not use kvm_write_guest_cached() here and introduce kvm_read_guest_cached()
>>>>> for the read above?
>>>>
>>>> Actually, I'd expect most read/writes to benefit from caching, no?
>>>> So why don't we just rename kvm_write_guest_cached() to
>>>> kvm_write_guest(), and the few places - if any - that need to force
>>>> transversing of the gfn mappings, get renamed to
>>>> kvm_write_guest_uncached ?
>>>>
>>> Good idea. I do not see any places where kvm_write_guest_uncached is
>>> needed from a brief look. Avi?
>>>
>>>>>> + &vcpu->arch.st.steal, sizeof(struct kvm_steal_time)))) {
>>>>>> +
>>>>>> + vcpu->arch.st.stime = 0;
>>>>>> + return;
>>>>>> + }
>>>>>> + }
>>>>>> +
>>>>>> +}
>>>>>> +
>>>>>> int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>>>>>> {
>>>>>> switch (msr) {
>>>>>> @@ -1562,6 +1590,23 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
>>>>>> if (kvm_pv_enable_async_pf(vcpu, data))
>>>>>> return 1;
>>>>>> break;
>>>>>> + case MSR_KVM_STEAL_TIME:
>>>>>> + vcpu->arch.st.msr_val = data;
>>>>>> +
>>>>>> + if (!(data& KVM_MSR_ENABLED)) {
>>>>>> + vcpu->arch.st.stime = 0;
>>>>>> + break;
>>>>>> + }
>>>>>> +
>>>>>> + if (data& KVM_STEAL_RESERVED_MASK)
>>>>>> + return 1;
>>>>>> +
>>>>>> + vcpu->arch.st.this_time_out = get_kernel_ns();
>>>>>> + vcpu->arch.st.stime = data& KVM_STEAL_VALID_BITS;
>>>>>> + record_steal_time(vcpu);
>>>>>> +
>>>>>> + break;
>>>>>> +
>>>>>> case MSR_IA32_MCG_CTL:
>>>>>> case MSR_IA32_MCG_STATUS:
>>>>>> case MSR_IA32_MC0_CTL ... MSR_IA32_MC0_CTL + 4 * KVM_MAX_MCE_BANKS - 1:
>>>>>> @@ -1847,6 +1892,9 @@ int kvm_get_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 *pdata)
>>>>>> case MSR_KVM_ASYNC_PF_EN:
>>>>>> data = vcpu->arch.apf.msr_val;
>>>>>> break;
>>>>>> + case MSR_KVM_STEAL_TIME:
>>>>>> + data = vcpu->arch.st.msr_val;
>>>>>> + break;
>>>>>> case MSR_IA32_P5_MC_ADDR:
>>>>>> case MSR_IA32_P5_MC_TYPE:
>>>>>> case MSR_IA32_MCG_CAP:
>>>>>> @@ -2158,6 +2206,8 @@ void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
>>>>>> kvm_migrate_timers(vcpu);
>>>>>> vcpu->cpu = cpu;
>>>>>> }
>>>>>> +
>>>>>> + record_steal_time(vcpu);
>>>>>> }
>>>>>>
>>>>>> void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>>>>> @@ -2165,6 +2215,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>>>>> kvm_x86_ops->vcpu_put(vcpu);
>>>>>> kvm_put_guest_fpu(vcpu);
>>>>>> kvm_get_msr(vcpu, MSR_IA32_TSC,&vcpu->arch.last_guest_tsc);
>>>>>> + vcpu->arch.st.this_time_out = get_kernel_ns();
>>>>>> }
>>>>>>
>>>>> Shouldn't we call record_steal_time(vcpu)/vcpu->arch.st.this_time_out = get_kernel_ns();
>>>>> just before/after entering/exiting a guest? vcpu_(put|get) are called
>>>>> for each vcpu ioctl, not only VCPU_RUN.
>>>> Sorry, missed that the first time I've read your e-mail.
>>>>
>>>> If done like you said, time spent on the hypervisor is accounted as
>>>> steal time. I don't think it is.
>>> I thought that this is the point of a steal time. Running other
>>> tasks/guests is a hypervisor overhead too after all :) Also what about
>>> time spend serving host interrupts between put/get? It will not be
>>> accounted as steal time, correct?
>>
>> This is mostly semantics. I like to compare this to a normal
>> process: There is a difference between time the OS spent on your
>> behalf, doing your system calls (sys), and time spent by other
>> processes. Similar thing here.
>>
> The problem with this approach is that things like doing "info cpus"
> in qemu monitor will change guest scheduling behaviour. Do we want it
> to be like that?

You mean because it is running in a different thread, and will compete
for resources with the cpu thread ?

>> Which put/get are you referring to specifically ? You mean
>> vcpu_put() vs vcpu_load() ?
>>
> Yes.
>
>> If they are after vcpu_put(), they will, because at this time your
>> process is officially out of the cpu.
>>
> And if they are between vcpu_load() and vcpu_put() they will be accounted as
> a work done on behalf of a guest although they are likely unrelated.
I think the best we can do around it here is record steal time / measure
time as late as we can. We could in theory subtract irq time, but it
sounds too complicated for little gain.

>>
>>>>
>>>> Steal time is time spent running someone else's job instead of
>>>> yours. The name for the time spent in the hypervisor doing something
>>>> for *you* is just overhead.
>>> OK. That is the question of a definition I guess. If you define it like
>>> that the code is correct.
>>>
>>>>
>>>>>
>>>>>> static int is_efer_nx(void)
>>>>>> @@ -2477,7 +2528,8 @@ static void do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>>>>>> (1<< KVM_FEATURE_NOP_IO_DELAY) |
>>>>>> (1<< KVM_FEATURE_CLOCKSOURCE2) |
>>>>>> (1<< KVM_FEATURE_ASYNC_PF) |
>>>>>> - (1<< KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
>>>>>> + (1<< KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
>>>>>> + (1<< KVM_FEATURE_STEAL_TIME);
>>>>>> entry->ebx = 0;
>>>>>> entry->ecx = 0;
>>>>>> entry->edx = 0;
>>>>>> @@ -6200,6 +6252,8 @@ int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)
>>>>>>
>>>>>> kvmclock_reset(vcpu);
>>>>>>
>>>>>> + vcpu->arch.st.stime = 0;
>>>>>> +
>>>>>> kvm_clear_async_pf_completion_queue(vcpu);
>>>>>> kvm_async_pf_hash_reset(vcpu);
>>>>>> vcpu->arch.apf.halted = false;
>>>>>> --
>>>>>> 1.7.3.4
>>>>>>
>>>>>> --
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>>>>>
>>>>> --
>>>>> Gleb.
>>>
>>> --
>>> Gleb.
>
> --
> Gleb.