Re: [RFC] vduse config write support

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Jul 26, 2024 at 12:33:24PM +0530, Srivatsa Vaddagiri wrote:
> > Maybe you can give an example for this? We cache the configuration
> > space to a faster access to that.
> 
> Yes for the same reason of faster access, we wish to have config information
> cached in hypervisor. In case of VDUSE,
> 
> Guest read -> (VM exit) -> VHOST_VDPA_GET_CONFIG -> vduse_vdpa_get_config
> 
> Basically a guest read terminates in host kernel, before resuming.
> 
> In our case,
> 
> Guest read - (VM exit) - Hyp emulates read - (VM resume)
> 
> So a guest read would terminate in hypervisor itself, before it is resumed. 
> 
> Without this optimization, guest VCPU would have stalled until VMM in host can
> emulate it, which can be long, especially a concern when the read is issued in
> hot path (interrupt handler, w/o MSI_X).

Instead of optimizing INT#x (which is futile, it's a shared interrupt
and will trigger a ton of exits anyway), just enable MSI-X.