Re: [Qemu-ppc] [PATCH v3 03/15] spapr/xive: add hcall support when under KVM

[David Gibson]

On Thu, Mar 21, 2019 at 03:49:02PM +0100, Cédric Le Goater wrote:
> XIVE hcalls are all redirected to QEMU as none are on a fast path.
> When necessary, QEMU invokes KVM through specific ioctls to perform
> host operations. QEMU should have done the necessary checks before
> calling KVM and, in case of failure, H_HARDWARE is simply returned.
> 
> H_INT_ESB is a special case that could have been handled under KVM
> but the impact on performance was low when under QEMU. Here are some
> figures :
> 
>     kernel irqchip      OFF          ON
>     H_INT_ESB                    KVM   QEMU
> 
>     rtl8139 (LSI )      1.19     1.24  1.23  Gbits/sec
>     virtio             31.80    42.30   --   Gbits/sec
> 
> Signed-off-by: Cédric Le Goater <address@hidden>

Reviewed-by: David Gibson <address@hidden>

> ---
> 
>  What has not changed :
> 
>  - Memory barriers have not been included. Only loads are performed on
>    the ESB management page (no stores for now) and stores are done
>    only on the trigger page. We should be fine as for the load
>    ordering on the management page.

I'm not entirely sure I'm convinced by this reasoning, but I'll take
your word for it.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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