Re: [PATCH v6 3/8] KVM: Add support for using dirty ring in conjunction with bitmap

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On Mon, Oct 24, 2022, Sean Christopherson wrote:
> On Sat, Oct 22, 2022, Marc Zyngier wrote:
> > On Fri, 21 Oct 2022 17:05:26 +0100, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
> > > 
> > > On Fri, Oct 21, 2022, Marc Zyngier wrote:
> > > > Because dirtying memory outside of a vcpu context makes it
> > > > incredibly awkward to handle a "ring full" condition?
> > > 
> > > Kicking all vCPUs with the soft-full request isn't _that_ awkward.
> > > It's certainly sub-optimal, but if inserting into the per-VM ring is
> > > relatively rare, then in practice it's unlikely to impact guest
> > > performance.
> > 
> > But there is *nothing* to kick here. The kernel is dirtying pages,
> > devices are dirtying pages (DMA), and there is no context associated
> > with that. Which is why a finite ring is the wrong abstraction.
> 
> I don't follow.  If there's a VM, KVM can always kick all vCPUs.  Again, might
> be far from optimal, but it's an option.  If there's literally no VM, then KVM
> isn't involved at all and there's no "ring vs. bitmap" decision.

Finally caught up in the other part of the thread that calls out that the devices
can't be stalled.

https://lore.kernel.org/all/87czakgmc0.wl-maz@xxxxxxxxxx



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