Re: [Qemu-devel] [PATCH v3] spapr: compute default value of "hpt-max-page-size" later

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Jul 02, 2018 at 10:54:56AM +0200, Greg Kurz wrote:
> It is currently not possible to run a pseries-2.12 or older machine
> with HV KVM. QEMU prints the following and exits right away.
> 
> qemu-system-ppc64: KVM doesn't support for base page shift 34
> 
> The "hpt-max-page-size" capability was recently added to spapr to hide
> host configuration details from HPT mode guests. Its default value for
> newer machine types is 64k.
> 
> For backwards compatibility, pseries-2.12 and older machine types need
> a different value. This is handled as usual in a class init function.
> The default value is 16G, ie, all page sizes supported by POWER7 and
> newer CPUs, but HV KVM requires guest pages to be hpa contiguous as
> well as gpa contiguous. The default value is the page size used to
> back the guest RAM in this case.
> 
> Unfortunately kvmppc_hpt_needs_host_contiguous_pages()->kvm_enabled() is
> called way before KVM init and returns false, even if the user requested
> KVM. We thus end up selecting 16G, which isn't supported by HV KVM. The
> default value must be set during machine init, because we can safely
> assume that KVM is initialized at this point.
> 
> We fix this by moving the logic to default_caps_with_cpu(). Since the
> user cannot pass cap-hpt-max-page-size=0, we set the default to 0 in
> the pseries-2.12 class init function and use that as a flag to do the
> real work.
> 
> Signed-off-by: Greg Kurz <groug@kaod.org>

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>

-- 
Eduardo