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Date: Tue, 18 Jun 2019 08:25:35 -0700
From: Ram Pai <linuxram@us.ibm.com>
To: Segher Boessenkool <segher@kernel.crashing.org>
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Subject: Re: Re: [PATCH v3 4/9] KVM: PPC: Ultravisor: Add generic ultravisor
 call handler
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Cc: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>,
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On Tue, Jun 18, 2019 at 06:47:01AM -0500, Segher Boessenkool wrote:
> Hi Paul,
> 
> On Mon, Jun 17, 2019 at 12:06:32PM +1000, Paul Mackerras wrote:
> > The thing we need to consider is that when SMFCTRL[E] = 0, a ucall
> > instruction becomes a hcall (that is, sc 2 is executed as if it was
> > sc 1).  In that case, the first argument to the ucall will be
> > interpreted as the hcall number.  Mostly that will happen not to be a
> > valid hcall number, but sometimes it might unavoidably be a valid but
> > unintended hcall number.
> 
> Shouldn't a caller of the ultravisor *know* that it is talking to the
> ultravisor in the first place?  And not to the hypervisor.

It may or may not.  But if it knows and still decides to make the ucall,
   the hypervisor must gracefully handle it.  
   
 We can't control who makes a ucall. A normal process within the VM could
 make a ucall too. Or a normal process running on top of the hypervisor
 could make a ucall.

RP