Re: [PATCH v2] 9p/xen: increase XEN_9PFS_RING_ORDER

[Dominique Martinet <asmadeus@xxxxxxxxxxxxx>]

Stefano Stabellini wrote on Thu, May 21, 2020:
> From: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>
> 
> Increase XEN_9PFS_RING_ORDER to 9 for performance reason. Order 9 is the
> max allowed by the protocol.
> 
> We can't assume that all backends will support order 9. The xenstore
> property max-ring-page-order specifies the max order supported by the
> backend. We'll use max-ring-page-order for the size of the ring.
> 
> This means that the size of the ring is not static
> (XEN_FLEX_RING_SIZE(9)) anymore. Change XEN_9PFS_RING_SIZE to take an
> argument and base the calculation on the order chosen at setup time.
> 
> Finally, modify p9_xen_trans.maxsize to be divided by 4 compared to the
> original value. We need to divide it by 2 because we have two rings
> coming off the same order allocation: the in and out rings. This was a
> mistake in the original code. Also divide it further by 2 because we
> don't want a single request/reply to fill up the entire ring. There can
> be multiple requests/replies outstanding at any given time and if we use
> the full ring with one, we risk forcing the backend to wait for the
> client to read back more replies before continuing, which is not
> performant.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxx>

LGTM, I'll try to find some time to test this by the end of next week or
will trust you if I can't make it -- ping me around June 1st if I don't
reply again until then...

Cheers,
-- 
Dominique