Re: [Qemu-devel] [PATCH v2 0/7] kvm: Get coalesced MMIO flushing out of the hot-path

[Avi Kivity]

On 06/27/2012 07:27 PM, Jan Kiszka wrote:
> Changes in v2:
>  - added memory_region_clear_flush_coalesced
>  - call memory_region_clear_flush_coalesced from
>    memory_region_clear_coalescing
>  - wrap all region manipulations via memory_region_transaction_begin/
>    commit internally
>  - flush coalesced MMIO only on memory_region_transaction_begin
> 
> Original description:
> 
> We currently flush the coalesced MMIO buffer on every vmexit to
> userspace. KVM only provides a single buffer per VM, so a central lock
> is required to read from it. This is a contention point given a large
> enough VCPU set. Moreover, we need to hold the BQL while replaying the
> queued requests, probably for a long time until there is more fine
> grained locking available. Good reasons to overcome the unconditional
> flush.
> 
> The series achieves this by flushing only on selected memory region
> accesses, either generically via the memory access dispatcher or
> directly on certain VGA PIO accesses that are not yet fully converted.
> Another reason to flush are remappings or other relevant region state
> changes.

Looks good; please see minor comment on patch 2.

-- 
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