Re: [Qemu-devel] [PATCH qemu] spapr/target-ppc/kvm: Only add hcall-instructions if KVM supports it

[Alexander Graf <agraf@suse.de>]

On 15.03.16 06:51, Alexey Kardashevskiy wrote:
> ePAPR defines "hcall-instructions" device-tree property which contains
> code to call hypercalls in ePAPR paravirtualized guests. However this
> property is also present for pseries guests where it does not make sense,
> even though it contains dummy code which simply fails.
> 
> Instead of maintaining the property (which used to be BE only; then was
> fixed to be endian-agnostic) and confusing the guest (which might think
> there is ePAPR host while there is none), this simply does not
> the property to the device tree if the host kernel does not implement it.
> 
> In order to tell the machine code if the host kernel supports
> KVM_CAP_PPC_GET_PVINFO, this changes kvmppc_get_hypercall() to return 1
> if the host kernel does not implement it (which is HV KVM case).
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> 
> 
> Alexander,
> 
> We just got a bug report that LE guests would not boot under quite old QEMU
> and we (powerkvm) wonder if it makes sense to backport endian-agnostic
> hypercall code to older QEMU or it is simpler/more correct
> not to have epapr-hypercall property in the tree.

Without the property you lose KVM hypercalls, so mostly some PR
speedups. For HV KVM, I don't think it makes a lot of sense to expose
KVM specific hypercalls, but I'm not sure it's a great idea to block the
path. With the infrastructure in place, we can at least add non-sPAPR PV
if we want to.


Alex