Re: [PATCH RFC 04/15] KVM: Implement ring-based dirty memory tracking

[Peter Xu]

On Mon, Dec 02, 2019 at 12:10:36PM -0800, Sean Christopherson wrote:
> On Fri, Nov 29, 2019 at 04:34:54PM -0500, Peter Xu wrote:
> > This patch is heavily based on previous work from Lei Cao
> > <lei.cao@xxxxxxxxxxx> and Paolo Bonzini <pbonzini@xxxxxxxxxx>. [1]
> > 
> > KVM currently uses large bitmaps to track dirty memory.  These bitmaps
> > are copied to userspace when userspace queries KVM for its dirty page
> > information.  The use of bitmaps is mostly sufficient for live
> > migration, as large parts of memory are be dirtied from one log-dirty
> > pass to another.  However, in a checkpointing system, the number of
> > dirty pages is small and in fact it is often bounded---the VM is
> > paused when it has dirtied a pre-defined number of pages. Traversing a
> > large, sparsely populated bitmap to find set bits is time-consuming,
> > as is copying the bitmap to user-space.
> > 
> > A similar issue will be there for live migration when the guest memory
> > is huge while the page dirty procedure is trivial.  In that case for
> > each dirty sync we need to pull the whole dirty bitmap to userspace
> > and analyse every bit even if it's mostly zeros.
> > 
> > The preferred data structure for above scenarios is a dense list of
> > guest frame numbers (GFN).  This patch series stores the dirty list in
> > kernel memory that can be memory mapped into userspace to allow speedy
> > harvesting.
> > 
> > We defined two new data structures:
> > 
> >   struct kvm_dirty_ring;
> >   struct kvm_dirty_ring_indexes;
> > 
> > Firstly, kvm_dirty_ring is defined to represent a ring of dirty
> > pages.  When dirty tracking is enabled, we can push dirty gfn onto the
> > ring.
> > 
> > Secondly, kvm_dirty_ring_indexes is defined to represent the
> > user/kernel interface of each ring.  Currently it contains two
> > indexes: (1) avail_index represents where we should push our next
> > PFN (written by kernel), while (2) fetch_index represents where the
> > userspace should fetch the next dirty PFN (written by userspace).
> > 
> > One complete ring is composed by one kvm_dirty_ring plus its
> > corresponding kvm_dirty_ring_indexes.
> > 
> > Currently, we have N+1 rings for each VM of N vcpus:
> > 
> >   - for each vcpu, we have 1 per-vcpu dirty ring,
> >   - for each vm, we have 1 per-vm dirty ring
> 
> Why?  I assume the purpose of per-vcpu rings is to avoid contention between
> threads, but the motiviation needs to be explicitly stated.  And why is a
> per-vm fallback ring needed?

Yes, as explained in previous reply, the problem is there could have
guest memory writes without vcpu contexts.

> 
> If my assumption is correct, have other approaches been tried/profiled?
> E.g. using cmpxchg to reserve N number of entries in a shared ring.

Not yet, but I'd be fine to try anything if there's better
alternatives.  Besides, could you help explain why sharing one ring
and let each vcpu to reserve a region in the ring could be helpful in
the pov of performance?

> IMO,
> adding kvm_get_running_vcpu() is a hack that is just asking for future
> abuse and the vcpu/vm/as_id interactions in mark_page_dirty_in_ring()
> look extremely fragile.

I agree.  Another way is to put heavier traffic to the per-vm ring,
but the downside could be that the per-vm ring could get full easier
(but I haven't tested).

> I also dislike having two different mechanisms
> for accessing the ring (lock for per-vm, something else for per-vcpu).

Actually I proposed to drop the per-vm ring (actually I had a version
that implemented this.. and I just changed it back to the per-vm ring
later on, see below) and when there's no vcpu context I thought about:

  (1) use vcpu0 ring

  (2) or a better algo to pick up a per-vcpu ring (like, the less full
      ring, we can do many things here, e.g., we can easily maintain a
      structure track this so we can get O(1) search, I think)

I discussed this with Paolo, but I think Paolo preferred the per-vm
ring because there's no good reason to choose vcpu0 as what (1)
suggested.  While if to choose (2) we probably need to lock even for
per-cpu ring, so could be a bit slower.

Since this is still RFC, I think we still have chance to change this,
depending on how the discussion goes.

> 
> > Please refer to the documentation update in this patch for more
> > details.
> > 
> > Note that this patch implements the core logic of dirty ring buffer.
> > It's still disabled for all archs for now.  Also, we'll address some
> > of the other issues in follow up patches before it's firstly enabled
> > on x86.
> > 
> > [1] https://patchwork.kernel.org/patch/10471409/
> > 
> > Signed-off-by: Lei Cao <lei.cao@xxxxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> > Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
> > ---
> 
> ...
> 
> > diff --git a/virt/kvm/dirty_ring.c b/virt/kvm/dirty_ring.c
> > new file mode 100644
> > index 000000000000..9264891f3c32
> > --- /dev/null
> > +++ b/virt/kvm/dirty_ring.c
> > @@ -0,0 +1,156 @@
> > +#include <linux/kvm_host.h>
> > +#include <linux/kvm.h>
> > +#include <linux/vmalloc.h>
> > +#include <linux/kvm_dirty_ring.h>
> > +
> > +u32 kvm_dirty_ring_get_rsvd_entries(void)
> > +{
> > +	return KVM_DIRTY_RING_RSVD_ENTRIES + kvm_cpu_dirty_log_size();
> > +}
> > +
> > +int kvm_dirty_ring_alloc(struct kvm *kvm, struct kvm_dirty_ring *ring)
> > +{
> > +	u32 size = kvm->dirty_ring_size;
> 
> Just pass in @size, that way you don't need @kvm.  And the callers will be
> less ugly, e.g. the initial allocation won't need to speculatively set
> kvm->dirty_ring_size.

Sure.

> 
> > +
> > +	ring->dirty_gfns = vmalloc(size);
> > +	if (!ring->dirty_gfns)
> > +		return -ENOMEM;
> > +	memset(ring->dirty_gfns, 0, size);
> > +
> > +	ring->size = size / sizeof(struct kvm_dirty_gfn);
> > +	ring->soft_limit =
> > +	    (kvm->dirty_ring_size / sizeof(struct kvm_dirty_gfn)) -
> 
> And passing @size avoids issues like this where a local var is ignored.
> 
> > +	    kvm_dirty_ring_get_rsvd_entries();
> > +	ring->dirty_index = 0;
> > +	ring->reset_index = 0;
> > +	spin_lock_init(&ring->lock);
> > +
> > +	return 0;
> > +}
> > +
> 
> ...
> 
> > +void kvm_dirty_ring_free(struct kvm_dirty_ring *ring)
> > +{
> > +	if (ring->dirty_gfns) {
> 
> Why condition freeing the dirty ring on kvm->dirty_ring_size, this
> obviously protects itself.  Not to mention vfree() also plays nice with a
> NULL input.

Ok I can drop this check.

> 
> > +		vfree(ring->dirty_gfns);
> > +		ring->dirty_gfns = NULL;
> > +	}
> > +}
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 681452d288cd..8642c977629b 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -64,6 +64,8 @@
> >  #define CREATE_TRACE_POINTS
> >  #include <trace/events/kvm.h>
> >  
> > +#include <linux/kvm_dirty_ring.h>
> > +
> >  /* Worst case buffer size needed for holding an integer. */
> >  #define ITOA_MAX_LEN 12
> >  
> > @@ -149,6 +151,10 @@ static void mark_page_dirty_in_slot(struct kvm *kvm,
> >  				    struct kvm_vcpu *vcpu,
> >  				    struct kvm_memory_slot *memslot,
> >  				    gfn_t gfn);
> > +static void mark_page_dirty_in_ring(struct kvm *kvm,
> > +				    struct kvm_vcpu *vcpu,
> > +				    struct kvm_memory_slot *slot,
> > +				    gfn_t gfn);
> >  
> >  __visible bool kvm_rebooting;
> >  EXPORT_SYMBOL_GPL(kvm_rebooting);
> > @@ -359,11 +365,22 @@ int kvm_vcpu_init(struct kvm_vcpu *vcpu, struct kvm *kvm, unsigned id)
> >  	vcpu->preempted = false;
> >  	vcpu->ready = false;
> >  
> > +	if (kvm->dirty_ring_size) {
> > +		r = kvm_dirty_ring_alloc(vcpu->kvm, &vcpu->dirty_ring);
> > +		if (r) {
> > +			kvm->dirty_ring_size = 0;
> > +			goto fail_free_run;
> 
> This looks wrong, kvm->dirty_ring_size is used to free allocations, i.e.
> previous allocations will leak if a vcpu allocation fails.

You are right.  That's an overkill.

> 
> > +		}
> > +	}
> > +
> >  	r = kvm_arch_vcpu_init(vcpu);
> >  	if (r < 0)
> > -		goto fail_free_run;
> > +		goto fail_free_ring;
> >  	return 0;
> >  
> > +fail_free_ring:
> > +	if (kvm->dirty_ring_size)
> > +		kvm_dirty_ring_free(&vcpu->dirty_ring);
> >  fail_free_run:
> >  	free_page((unsigned long)vcpu->run);
> >  fail:
> > @@ -381,6 +398,8 @@ void kvm_vcpu_uninit(struct kvm_vcpu *vcpu)
> >  	put_pid(rcu_dereference_protected(vcpu->pid, 1));
> >  	kvm_arch_vcpu_uninit(vcpu);
> >  	free_page((unsigned long)vcpu->run);
> > +	if (vcpu->kvm->dirty_ring_size)
> > +		kvm_dirty_ring_free(&vcpu->dirty_ring);
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_vcpu_uninit);
> >  
> > @@ -690,6 +709,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
> >  	struct kvm *kvm = kvm_arch_alloc_vm();
> >  	int r = -ENOMEM;
> >  	int i;
> > +	struct page *page;
> >  
> >  	if (!kvm)
> >  		return ERR_PTR(-ENOMEM);
> > @@ -705,6 +725,14 @@ static struct kvm *kvm_create_vm(unsigned long type)
> >  
> >  	BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
> >  
> > +	page = alloc_page(GFP_KERNEL | __GFP_ZERO);
> > +	if (!page) {
> > +		r = -ENOMEM;
> > +		goto out_err_alloc_page;
> > +	}
> > +	kvm->vm_run = page_address(page);
> > +	BUILD_BUG_ON(sizeof(struct kvm_vm_run) > PAGE_SIZE);
> > +
> >  	if (init_srcu_struct(&kvm->srcu))
> >  		goto out_err_no_srcu;
> >  	if (init_srcu_struct(&kvm->irq_srcu))
> > @@ -775,6 +803,9 @@ static struct kvm *kvm_create_vm(unsigned long type)
> >  out_err_no_irq_srcu:
> >  	cleanup_srcu_struct(&kvm->srcu);
> >  out_err_no_srcu:
> > +	free_page((unsigned long)page);
> > +	kvm->vm_run = NULL;
> 
> No need to nullify vm_run.

Ok.

> 
> > +out_err_alloc_page:
> >  	kvm_arch_free_vm(kvm);
> >  	mmdrop(current->mm);
> >  	return ERR_PTR(r);
> > @@ -800,6 +831,15 @@ static void kvm_destroy_vm(struct kvm *kvm)
> >  	int i;
> >  	struct mm_struct *mm = kvm->mm;
> >  
> > +	if (kvm->dirty_ring_size) {
> > +		kvm_dirty_ring_free(&kvm->vm_dirty_ring);
> > +	}
> 
> Unnecessary parantheses.

True.

Thanks,

> 
> > +
> > +	if (kvm->vm_run) {
> > +		free_page((unsigned long)kvm->vm_run);
> > +		kvm->vm_run = NULL;
> > +	}
> > +
> >  	kvm_uevent_notify_change(KVM_EVENT_DESTROY_VM, kvm);
> >  	kvm_destroy_vm_debugfs(kvm);
> >  	kvm_arch_sync_events(kvm);
> > @@ -2301,7 +2341,7 @@ static void mark_page_dirty_in_slot(struct kvm *kvm,
> >  {
> >  	if (memslot && memslot->dirty_bitmap) {
> >  		unsigned long rel_gfn = gfn - memslot->base_gfn;
> > -
> > +		mark_page_dirty_in_ring(kvm, vcpu, memslot, gfn);
> >  		set_bit_le(rel_gfn, memslot->dirty_bitmap);
> >  	}
> >  }
> > @@ -2649,6 +2689,13 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
> >  }
> >  EXPORT_SYMBOL_GPL(kvm_vcpu_on_spin);
> 

-- 
Peter Xu