Re: KVM & VT-d2?

[Avi Kivity <avi@redhat.com>]

Fischer, Anna wrote:
> I thought that one use case of VT-d2 interrupt remapping was to be able to safely and more efficiently deliver interrupts to the CPU that runs the particular VCPU of the guest that owns the I/O device that issues the interrupt. Shouldn't there at least be some performance (e.g. latency) improvement doing the remapping and checking in HW with a predefined table rather than multiplexing this in software in the hypervisor layer?
>   

This can be done even without VT-d.  Since qemu knows how the guest 
affines its interrupts (via the MSI entry), it can affine the 
corresponding host interrupt.

Right now Linux can affine the host interrupt to a cpu; this requires 
periodic polling since the vcpu thread can move around.  It would be 
interesting to add an API to affine an interrupt to a thread, so 
whenever the thread is migrated to a different cpu, the interrupt 
follows it around.  I imagine it would be useful for non-virtualization 
applications as well.

-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.