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From: Peter Xu <peterx@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	qemu-devel@nongnu.org, "Michael S . Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH RFC 8/9] KVM: Add dirty-ring-size property
Date: Thu, 26 Mar 2020 12:03:26 -0400
Message-ID: <20200326160326.GF422390@xz-x1> (raw)
In-Reply-To: <20200326134144.GC2713@work-vm>

On Thu, Mar 26, 2020 at 01:41:44PM +0000, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > On Wed, Mar 25, 2020 at 08:00:31PM +0000, Dr. David Alan Gilbert wrote:
> > > > @@ -2077,6 +2079,33 @@ static int kvm_init(MachineState *ms)
> > > >      s->memory_listener.listener.coalesced_io_add = kvm_coalesce_mmio_region;
> > > >      s->memory_listener.listener.coalesced_io_del = kvm_uncoalesce_mmio_region;
> > > >  
> > > > +    /*
> > > > +     * Enable KVM dirty ring if supported, otherwise fall back to
> > > > +     * dirty logging mode
> > > > +     */
> > > > +    if (s->kvm_dirty_ring_size > 0) {
> > > > +        /* Read the max supported pages */
> > > > +        ret = kvm_vm_check_extension(kvm_state, KVM_CAP_DIRTY_LOG_RING);
> > > > +        if (ret > 0) {
> > > > +            if (s->kvm_dirty_ring_size > ret) {
> > > > +                error_report("KVM dirty ring size %d too big (maximum is %d). "
> > > > +                             "Please use a smaller value.",
> > > > +                             s->kvm_dirty_ring_size, ret);
> > > > +                goto err;
> > > > +            }
> > > > +
> > > > +            ret = kvm_vm_enable_cap(s, KVM_CAP_DIRTY_LOG_RING, 0,
> > > > +                                    s->kvm_dirty_ring_size);
> > > > +            if (ret) {
> > > > +                error_report("Enabling of KVM dirty ring failed: %d", ret);
> > > > +                goto err;
> > > > +            }
> > > > +
> > > > +            s->kvm_dirty_gfn_count =
> > > > +                s->kvm_dirty_ring_size / sizeof(struct kvm_dirty_gfn);
> > > 
> > > What happens if I was to pass dirty-ring-size=1 ?
> > > Then the count would be 0 and things would get upset somewhere?
> > > Do you need to check for a minimum postive value?
> > 
> > The above kvm_vm_enable_cap() should fail directly and QEMU will stop.
> > Yes we should check it, but kernel will do that in all cases, so I
> > just didn't do that explicitly again in the userspace.
> 
> We probably should have that check since you can give them a more
> obvious error message.

Yes we can.  Or I can enhance the error message when we failed with
kvm_vm_enable_cap() so the user will get the important hints.  I think
maybe that's more important than the explicit check itself.

> 
> > I was planning this to be an advanced feature so the user of this
> > parameter should know the rules to pass values in.
> 
> Advanced users just make advanced mistakes :-)
> 
> I did wonder if perhaps this option should be a count of entries rather
> than a byte size.

Sometimes it's easier to know "how many bytes we used", while instead
sometimes we want to know "how many dirty addresses we can track".
But sure I can switch, considering the users might be more interested
in the latter. :)

Thanks,

-- 
Peter Xu



  reply index

Thread overview: 31+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-05 14:17 [PATCH RFC 0/9] KVM: Dirty ring support (QEMU part) Peter Xu
2020-02-05 14:17 ` [PATCH RFC 1/9] KVM: Fixup kvm_log_clear_one_slot() ioctl return check Peter Xu
2020-03-25 16:34   ` Dr. David Alan Gilbert
2020-03-25 17:43   ` Peter Xu
2020-02-05 14:17 ` [PATCH RFC 2/9] linux-headers: Update Peter Xu
2020-02-05 14:17 ` [PATCH RFC 3/9] memory: Introduce log_sync_global() to memory listener Peter Xu
2020-03-25 16:52   ` Dr. David Alan Gilbert
2020-03-25 17:02     ` Peter Xu
2020-02-05 14:17 ` [PATCH RFC 4/9] KVM: Create the KVMSlot dirty bitmap on flag changes Peter Xu
2020-03-25 17:44   ` Dr. David Alan Gilbert
2020-02-05 14:17 ` [PATCH RFC 5/9] KVM: Provide helper to get kvm dirty log Peter Xu
2020-03-25 17:52   ` Dr. David Alan Gilbert
2020-03-25 18:15     ` Peter Xu
2020-02-05 14:17 ` [PATCH RFC 6/9] KVM: Provide helper to sync dirty bitmap from slot to ramblock Peter Xu
2020-03-25 18:47   ` Dr. David Alan Gilbert
2020-02-05 14:17 ` [PATCH RFC 7/9] KVM: Cache kvm slot dirty bitmap size Peter Xu
2020-03-25 18:49   ` Dr. David Alan Gilbert
2020-02-05 14:17 ` [PATCH RFC 8/9] KVM: Add dirty-ring-size property Peter Xu
2020-03-25 20:00   ` Dr. David Alan Gilbert
2020-03-25 20:42     ` Peter Xu
2020-03-26 13:41       ` Dr. David Alan Gilbert
2020-03-26 16:03         ` Peter Xu [this message]
2020-03-25 20:14   ` Dr. David Alan Gilbert
2020-03-25 20:48     ` Peter Xu
2020-02-05 14:17 ` [PATCH RFC 9/9] KVM: Dirty ring support Peter Xu
2020-03-25 20:41   ` Dr. David Alan Gilbert
2020-03-25 21:32     ` Peter Xu
2020-03-26 14:14       ` Dr. David Alan Gilbert
2020-03-26 16:10         ` Peter Xu
2020-02-05 14:51 ` [PATCH RFC 0/9] KVM: Dirty ring support (QEMU part) no-reply
2020-03-03 17:32 ` Peter Xu

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