[PATCH 11/19] KVM: PPC: Book3S HV: add support for the XIVE native exploitation mode hcalls

[Benjamin Herrenschmidt]

On Wed, 2019-01-23 at 21:26 +1100, Paul Mackerras wrote:
> If H_INT_ESB is only used for LSIs, then is a guest going to be using
> it at all?  

*emulated* LSIs, ie LSIs coming from emulated devices. It will depends
in practice of what kind of emulated device you put in your guest. We
need that because under the hood, we send a XIVE MSI, so we need to be
notified of the EOI so we can resend if the emulated LSI is still
asserted. 

> My understanding was that with XIVE, only a small number
> of interrupts that are to do with system management functions are
> LSIs; all of the interrupts relating to PCI-e devices are MSIs.  So do
> we actually have a real high-frequency use case for LSIs in a guest?
> 
> For now I would prefer that you remove all the real-mode hcall
> handlers.  We can add them later if we get performance data showing
> that they are needed.
> 
> Regarding whether or not to have a given hcall handler in the kernel
> at all - if there is for example an hcall which is just called once
> on guest startup, and its function is just to provide information to
> the guest, and QEMU has that information, then why not have that hcall
> implemented by QEMU?  Are any of the hcalls like that?
> 
> For example, if H_INT_GET_SOURCE_INFO was implemented in QEMU, could
> we then remove the VC_BASE thing from the xive device?

Ben.