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Subject: Re: [PATCH 11/19] KVM: PPC: Book3S HV: add support for the XIVE
 native exploitation mode hcalls
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Paul Mackerras <paulus@ozlabs.org>, =?ISO-8859-1?Q?C=E9dric?= Le Goater
 <clg@kaod.org>
Date: Thu, 24 Jan 2019 08:23:52 +1100
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Cc: linuxppc-dev@lists.ozlabs.org, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org,
 David Gibson <david@gibson.dropbear.id.au>
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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.