Re: [PATCH RFC 0/9] KVM: Dirty ring support (QEMU part)

[Peter Xu]

On Wed, Feb 05, 2020 at 09:17:40AM -0500, Peter Xu wrote:
> This is still RFC because the kernel counterpart is still under
> review.  However please feel free to read into the code a bit if you
> want; they've even got rich comments so not really in WIP status
> itself.  Any kind of comments are greatly welcomed.
> 
> For anyone who wants to try (we need to upgrade kernel too):
> 
> KVM branch:
>   https://github.com/xzpeter/linux/tree/kvm-dirty-ring
> 
> QEMU branch for testing:
>   https://github.com/xzpeter/qemu/tree/kvm-dirty-ring
> 
> Overview
> ========
> 
> KVM dirty ring is a new interface to pass over dirty bits from kernel
> to the userspace.  Instead of using a bitmap for each memory region,
> the dirty ring contains an array of dirtied GPAs to fetch, one ring
> per vcpu.
> 
> There're a few major changes comparing to how the old dirty logging
> interface would work:
> 
> - Granularity of dirty bits
> 
>   KVM dirty ring interface does not offer memory region level
>   granularity to collect dirty bits (i.e., per KVM memory
>   slot). Instead the dirty bit is collected globally for all the vcpus
>   at once.  The major effect is on VGA part because VGA dirty tracking
>   is enabled as long as the device is created, also it was in memory
>   region granularity.  Now that operation will be amplified to a VM
>   sync.  Maybe there's smarter way to do the same thing in VGA with
>   the new interface, but so far I don't see it affects much at least
>   on regular VMs.
> 
> - Collection of dirty bits
> 
>   The old dirty logging interface collects KVM dirty bits when
>   synchronizing dirty bits.  KVM dirty ring interface instead used a
>   standalone thread to do that.  So when the other thread (e.g., the
>   migration thread) wants to synchronize the dirty bits, it simply
>   kick the thread and wait until it flushes all the dirty bits to the
>   ramblock dirty bitmap.
> 
> A new parameter "dirty-ring-size" is added to "-accel kvm".  By
> default, dirty ring is still disabled (size==0).  To enable it, we
> need to be with:
> 
>   -accel kvm,dirty-ring-size=65536
> 
> This establishes a 64K dirty ring buffer per vcpu.  Then if we
> migrate, it'll switch to dirty ring.

Ping?

I'd like to know whether there's any high level comment about all
these... Considering that the kernel counterpart is still during
review, I guess we don't need to spend much time on that much,
assuming it'll be a ring of dirty addresses.  The rest should be
irrelevant to kernel so I'd glad to know if there's comments on those
parts.

Thanks,

-- 
Peter Xu