Re: [PATCH v2 0/6] KVM: Improve MMIO Coalescing API

[Sean Christopherson <seanjc@google.com>]

On Thu, Jul 18, 2024, Ilias Stamatis wrote:
> The current MMIO coalescing design has a few drawbacks which limit its
> usefulness. Currently all coalesced MMIO zones use the same ring buffer.
> That means that upon a userspace exit we have to handle potentially
> unrelated MMIO writes synchronously. And a VM-wide lock needs to be
> taken in the kernel when an MMIO exit occurs.
> 
> Additionally, there is no direct way for userspace to be notified about
> coalesced MMIO writes. If the next MMIO exit to userspace is when the
> ring buffer has filled then a substantial (and unbounded) amount of time
> may have passed since the first coalesced MMIO.
> 
> This series adds new ioctls to KVM that allow for greater control by
> making it possible to associate different MMIO zones with different ring
> buffers. It also allows userspace to use poll() to check for coalesced
> writes in order to avoid userspace exits in vCPU threads (see patch 3
> for why this can be useful).
> 
> The idea of improving the API in this way originally came from Paul
> Durrant (pdurrant@amazon.co.uk) but the implementation is mine.
> 
> The first patch in the series is a bug in the existing code that I
> discovered while writing a selftest and can be merged independently.

Ya, I'll grab it, maybe for 6.11?  Doesn't seem urgent though.

Anyways, I gave this a *very* cursory review.  I'd go ahead and send v3 though,
you're going to need Paolo's eyeballs on this, and he's offline until September.