Re: [PATCH RFC 9/9] KVM: Dirty ring support

[Peter Xu]

On Thu, Mar 26, 2020 at 02:14:36PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (address@hidden) wrote:
> > On Wed, Mar 25, 2020 at 08:41:44PM +0000, Dr. David Alan Gilbert wrote:
> > 
> > [...]
> > 
> > > > +enum KVMReaperState {
> > > > +    KVM_REAPER_NONE = 0,
> > > > +    /* The reaper is sleeping */
> > > > +    KVM_REAPER_WAIT,
> > > > +    /* The reaper is reaping for dirty pages */
> > > > +    KVM_REAPER_REAPING,
> > > > +};
> > > 
> > > That probably needs to be KVMDirtyRingReaperState
> > > given there are many things that could be reaped.
> > 
> > Sure.
> > 
> > > 
> > > > +/*
> > > > + * KVM reaper instance, responsible for collecting the KVM dirty bits
> > > > + * via the dirty ring.
> > > > + */
> > > > +struct KVMDirtyRingReaper {
> > > > +    /* The reaper thread */
> > > > +    QemuThread reaper_thr;
> > > > +    /*
> > > > +     * Telling the reaper thread to wakeup.  This should be used as a
> > > > +     * generic interface to kick the reaper thread, like, in vcpu
> > > > +     * threads where it gets a exit due to ring full.
> > > > +     */
> > > > +    EventNotifier reaper_event;
> > > 
> > > I think I'd just used a simple semaphore for this type of thing.
> > 
> > I'm actually uncertain on which is cheaper...
> > 
> > At the meantime, I wanted to poll two handles at the same time below
> > (in kvm_dirty_ring_reaper_thread).  I don't know how to do that with
> > semaphore.  Could it?
> 
> If you're OK with EventNotifier stick with it;  it's just I'm used
> to doing with it with a semaphore; e.g. a flag then the semaphore - but
> that's fine.

Ah yes flags could work, though we probably need to be careful with
flags and use atomic accesses to avoid race conditions of flag lost.

Then I'll keep it, thanks.

> 
> > [...]
> > 
> > > > @@ -412,6 +460,18 @@ int kvm_init_vcpu(CPUState *cpu)
> > > >              (void *)cpu->kvm_run + s->coalesced_mmio * PAGE_SIZE;
> > > >      }
> > > >  
> > > > +    if (s->kvm_dirty_gfn_count) {
> > > > +        cpu->kvm_dirty_gfns = mmap(NULL, s->kvm_dirty_ring_size,
> > > > +                                   PROT_READ | PROT_WRITE, MAP_SHARED,
> > > 
> > > Is the MAP_SHARED required?
> > 
> > Yes it's required.  It's the same when we map the per-vcpu kvm_run.
> > 
> > If we use MAP_PRIVATE, it'll be in a COW fashion - when the userspace
> > writes to the dirty gfns the 1st time, it'll copy the current dirty
> > ring page in the kernel and from now on QEMU will never be able to see
> > what the kernel writes to the dirty gfn pages.  MAP_SHARED means the
> > userspace and the kernel shares exactly the same page(s).
> 
> OK, worth a comment.

Sure.

-- 
Peter Xu