Re: [PATCH v2] accel/qaic: Enable 1 MSI fallback mode

[Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>]

On Mon, Oct 16, 2023 at 11:00:36AM -0600, Jeffrey Hugo wrote:
> From: Carl Vanderlip <quic_carlv@quicinc.com>
> 
> Several virtualization use-cases either don't support 32 MultiMSIs
> (Xen/VMware) or have significant drawbacks to their use (KVM's vIOMMU,
> which is required to support 32 MSI, needs to allocate an alternate
> system memory space for each device using vIOMMU (e.g. 8GB VM mem and
> 2 cards => 8 + 2 * 8 = 24GB host memory required)). Support these
> cases by enabling a 1 MSI fallback mode.
> 
> Whenever all 32 MSIs requested are not available, a second request for
> a single MSI is made. Its success is the initiator of single MSI mode.
> This mode causes all interrupts generated by the device to be directed
> to the 0th MSI (firmware >=v1.10 will do this as a response to the PCIe
> MSI capability configuration). Likewise, all interrupt handlers for the
> device are registered to the 0th MSI.
> 
> Since the DBC interrupt handler checks if the DBC is in use or if
> there is any pending changes, the 'spurious' interrupts are
> disregarded. If there is work to be done, the standard threaded IRQ
> handler is dispatched.
> 
> On every interrupt, the MHI handler wakes up its threaded interrupt
> handler, and attempts to wake any waiters for MHI state events.
> 
> Performance is within +-0.6% for test cases that typify real world
> use. Larger differences ([-4,+132]%, avg +47%) exist for very simple
> tasks (e.g. addition) compiled for single NSPs. It is assumed that the
> small work and many interrupts typically cause contention (e.g. 16 NSPs
> vs 4 CPUs), as evidenced by the standard deviation between runs also
> decreasing (r=-0.48 between delta(Performace_test) and
> delta(StdDev_test/Avg_test))
> 
> Signed-off-by: Carl Vanderlip <quic_carlv@quicinc.com>
> Reviewed-by: Pranjal Ramajor Asha Kanojiya <quic_pkanojiy@quicinc.com>
> Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
> Signed-off-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Stanislaw Gruszka <stanislaw.gruszka@linux.intel.com>