Re: [RFC] para virt interface of perf to support kvm guest osstatistics collection in guest os

[Zhang, Yanmin]

On Wed, 2010-06-09 at 12:41 +0300, Avi Kivity wrote:
> On 06/09/2010 12:21 PM, Zhang, Yanmin wrote:
> >
> >> One thing that's missing is documentation of the guest/host ABI.  It
> >> will be a requirement for inclusion, but it will also be a great help
> >> for review, so please provide it ASAP.
> >>      
> > I will add such document. It will includes:
> > 1) Data struct perf_event definition. Guest os and host os have to share the same
> > data structure as host kernel will sync data (counte/overflows and others if needed)
> > changes back to guest os.
> > 2) A list of all hypercalls
> > 3) Guest need have NMI handler which checks all guest events.
> >    
> 
> Thanks.  It may be easier to have just the documentation for the first 
> few iterations so we can converge on a good interface, then update the 
> code to match the interface.
I thought over it last night. Your input is important. I need define a clear ABI.
At guest side, I plan to use perf_event->shadow to point to another data area, such like:
struct guest_perf_counter {
	u64 count;
	atomic_t overflows;
};

So host side just copy data to this area, then guest copy them to its own
perf_event.

The ABI becomes more simple than before. Function kvm_sync_event_to_guest also becomes
clearer. The ABI mostly includes the definition of struct perf_event_attr, guest_perf_counter,
and hypercalls.


> >> Disabling the watchdog is unfortunate.  Why is it necessary?
> >>      
> > perf always uses NMI, so we disable the nmi_watchdog when a perf_event is
> > set up in case they might have impact.
> >    
> 
> Ok.  Is that the case for the hardware pmus as well?  If so it might be 
> done in common code.
> 
> >>> +
> >>> +static int kvm_pmu_enable(struct perf_event *event)
> >>> +{
> >>> +	int ret;
> >>> +	unsigned long addr = __pa(event);
> >>> +
> >>> +	if (kvm_add_event(event))
> >>> +		return -1;
> >>> +
> >>> +	ret = kvm_hypercall3(KVM_PERF_OP, KVM_PERF_OP_ENABLE,
> >>> +	 		addr, (unsigned long) event->shadow);
> >>>
> >>>        
> >> This is suspicious.  Passing virtual addresses to the host is always
> >> problematic.  Or event->shadow only used as an opaque cookie?
> >>      
> > I use perf_event->shadow at both host and guest side.
> > 1) At host side, perf_event->shadow points to an area including the page
> > mapping information about guest perf_event. Host kernel uses it to sync data
> > changes back to guest;
> > 2) At guest side, perf_event->shadow save the virtual address of host
> > perf_event at host side. At guest side,
> > kvm_hypercall3(KVM_PERF_OP, KVM_PERF_OP_OPEN, ...) return the virtual address.
> > Guest os shouldn't use it but using it to pass it back to host kernel in following
> > hypercalls. It might be a security issue for host kernel. Originally, I planed guest
> > os not to use perf_event->shadow. Host kernel maintains a per-vcpu event-related
> > list whose key is addr of guest perf_event. But considering the vcpu thread migration
> > on logical cpu, such list needs lock and implementation becomes a little complicated.
> >
> > I will double-check the list method as the security issue is there.
> >    
> 
> Besides the other concern, you cannot live migrate a host virtual 
> address, since it changes from host to host.  It's better to use a 
> guest-chosen bounded small integer.
Ok. Perhaps a single u32 per guest os instance is enough. So I will change the shadow
pointing to a structure like below in guest kernel:

struct guest_perf_counter {
	u64 count;
	atomic_t overflows;
};

struct guest_perf_shadow {
	u32 id;
	struct guest_perf_counter sync_data;
};

atomic_t guest_perf_id; /*Global id counter per guest os*/


> 
> >> Need to detect the kvm pmu via its own cpuid bit.
> >>      
> > Ok. I will add a feature, KVM_FEATURE_PARA_PERF, something like
> > bit KVM_FEATURE_CLOCKSOURCE.
> >    
> 
> Don't forget Documentation/kvm/cpuid.txt.
Thanks for your kind reminder.

> >> Ok, so event->shadow is never dereferenced.  In that case better not
> >> make it a pointer at all, keep it unsigned long.
> >>      
> > Host kernel also uses it.
> >    
> 
> I see.  So put it in a union.  Or perhaps not even in a union - what if 
> a kvm guest is also acting as a kvm host?
My patch has consideration on it. I compiled kernel with host and guest support
at the same time. The accessing to perf_event->shadow is really under specific
scenarios, or they are just in specific functions. These functions are called
just bu host kernel , or just by guest kernel.

> 
> 
> >    
> >>      
> >>> +
> >>> +static void kvm_sync_event_to_guest(struct perf_event *event, int overflows)
> >>> +{
> >>> +	struct hw_perf_event *hwc =&event->hw;
> >>> +	struct kvmperf_event *kvmevent;
> >>> +	int offset, len, data_len, copied, page_offset;
> >>> +	struct page *event_page;
> >>> +	void *shared_kaddr;
> >>> +
> >>> +	kvmevent = event->shadow;
> >>> +	offset = kvmevent->event_offset;
> >>> +
> >>> +	/* Copy perf_event->count firstly */
> >>> +	offset += offsetof(struct perf_event, count);
> >>> +	if (offset<   PAGE_SIZE) {
> >>> +		event_page = kvmevent->event_page;
> >>> +		page_offset = offset;
> >>> +	} else {
> >>> +		event_page = kvmevent->event_page2;
> >>> +		page_offset = offset - PAGE_SIZE;
> >>> +	}
> >>> +	shared_kaddr = kmap_atomic(event_page, KM_USER0);
> >>> +	*((atomic64_t *)(shared_kaddr + page_offset)) = event->count;
> >>>
> >>>        
> >> This copy will not be atomic on 32-bit hosts.
> >>      
> > Right. But it shouldn't be a problem as vcpu thread stops when vmexit to
> > host to process events. In addition, only current cpu in guest accesses
> > perf_events linked to current cpu.
> >    
> 
> Ok.  These restrictions should be documented.
Perhaps I need write them down as code comments in the patch.

> 
> >>      
> >>> +}
> >>> +
> >>>
> >>> +static struct perf_event *
> >>> +kvm_pv_perf_op_open(struct kvm_vcpu *vcpu, gpa_t addr)
> >>> +{
> >>> +	int ret;
> >>> +	struct perf_event *event;
> >>> +	struct perf_event *host_event = NULL;
> >>> +	struct kvmperf_event *shadow = NULL;
> >>> +
> >>> +	event = kzalloc(sizeof(*event), GFP_KERNEL);
> >>> +	if (!event)
> >>> +		goto out;
> >>> +
> >>> +	shadow = kzalloc(sizeof(*shadow), GFP_KERNEL);
> >>> +	if (!shadow)
> >>> +		goto out;
> >>> +
> >>> +	shadow->event_page = gfn_to_page(vcpu->kvm, addr>>   PAGE_SHIFT);
> >>> +	shadow->event_offset = addr&   ~PAGE_MASK;
> >>>
> >>>        
> >> Might truncate on 32-bit hosts.  PAGE_MASK is 32-bit while addr is 64 bit.
> >>      
> > Above codes just run at host side. Is it possible that host kernel is 32 bit and
> > guest kernel is 64bits?
> 
> No, guest bitness always <= host bitness.  But gpa_t is 64-bit even on 
> 32-bit host/guest, so you can't use PAGE_MASK on that.
I will check it.

> 
> >>> +
> >>> +	ret = kvm_read_guest(vcpu->kvm, addr, event, sizeof(*event));
> >>> +	if (ret)
> >>> +		goto out;
> >>>
> >>>        
> >> I assume this is to check existence?
> >>      
> > Here calling kvm_read_guest is to get a copy of guest perf_event as it has
> > perf_event.attr. Host need the attr to create host perf_event.
> >
> >    
> >>    It doesn't work well with memory
> >> hotplug.  In general it's preferred to use
> >> kvm_read_guest()/kvm_write_guest() instead of gfn_to_page() during
> >> initialization to prevent pinning and allow for hotplug.
> >>      
> > That's an issue. But host kernel couldn't go to sleep when processing event
> > overflows under NMI context.
> >    
> 
> You can set a bit in vcpu->requests and schedule the copying there.  
> vcpu->requests is always checked before guest entry.
That becomes a little complicated as we need record overflowed events in vcpu.
Let me check it again.

> 
> 
> >
> >> It may be better to have the guest create an id to the host, and the
> >> host can simply look up the id in a table:
> >>      
> > Perhaps the address of guest perf_event is the best id.
> >    
> 
> That will need to be looked up in a hash table.  A small id is best 
> because it can be easily looked up by both guest and host.
Yes. I will use a u32 or atomic_t.

Thanks for your patience!

Yanmin


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