Re: [PATCH kvmtool v3 2/6] arm64: Initial nested virt support

[Marc Zyngier <maz@kernel.org>]

On Mon, 04 Aug 2025 15:41:48 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Andre,
> 
> On Tue, Jul 29, 2025 at 10:57:41AM +0100, Andre Przywara wrote:
> > The ARMv8.3 architecture update includes support for nested
> > virtualization. Allow the user to specify "--nested" to start a guest in
> > (virtual) EL2 instead of EL1.
> > This will also change the PSCI conduit from HVC to SMC in the device
> > tree.
> > 
> > Signed-off-by: Andre Przywara <andre.przywara@arm.com>
> > ---
> >  arm64/fdt.c                         |  5 ++++-
> >  arm64/include/kvm/kvm-config-arch.h |  5 ++++-
> >  arm64/kvm-cpu.c                     | 12 +++++++++++-
> >  3 files changed, 19 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arm64/fdt.c b/arm64/fdt.c
> > index df7775876..98f1dd9d4 100644
> > --- a/arm64/fdt.c
> > +++ b/arm64/fdt.c
> > @@ -205,7 +205,10 @@ static int setup_fdt(struct kvm *kvm)
> >  		_FDT(fdt_property_string(fdt, "compatible", "arm,psci"));
> >  		fns = &psci_0_1_fns;
> >  	}
> > -	_FDT(fdt_property_string(fdt, "method", "hvc"));
> > +	if (kvm->cfg.arch.nested_virt)
> > +		_FDT(fdt_property_string(fdt, "method", "smc"));
> > +	else
> > +		_FDT(fdt_property_string(fdt, "method", "hvc"));
> >  	_FDT(fdt_property_cell(fdt, "cpu_suspend", fns->cpu_suspend));
> >  	_FDT(fdt_property_cell(fdt, "cpu_off", fns->cpu_off));
> >  	_FDT(fdt_property_cell(fdt, "cpu_on", fns->cpu_on));
> > diff --git a/arm64/include/kvm/kvm-config-arch.h b/arm64/include/kvm/kvm-config-arch.h
> > index ee031f010..a1dac28e6 100644
> > --- a/arm64/include/kvm/kvm-config-arch.h
> > +++ b/arm64/include/kvm/kvm-config-arch.h
> > @@ -10,6 +10,7 @@ struct kvm_config_arch {
> >  	bool		aarch32_guest;
> >  	bool		has_pmuv3;
> >  	bool		mte_disabled;
> > +	bool		nested_virt;
> >  	u64		kaslr_seed;
> >  	enum irqchip_type irqchip;
> >  	u64		fw_addr;
> > @@ -57,6 +58,8 @@ int sve_vl_parser(const struct option *opt, const char *arg, int unset);
> >  		     "Type of interrupt controller to emulate in the guest",	\
> >  		     irqchip_parser, NULL),					\
> >  	OPT_U64('\0', "firmware-address", &(cfg)->fw_addr,			\
> > -		"Address where firmware should be loaded"),
> > +		"Address where firmware should be loaded"),			\
> > +	OPT_BOOLEAN('\0', "nested", &(cfg)->nested_virt,			\
> 
> --nested sounds a bit vague (what if KVM decides to nest something else in the
> future?) and the variable that keeps track of the parameter is called
> 'nested_virt'. Is it too late to rename --nested to --nested-virt for
> consistency and better clarity?

I disagree. 'nested' is pretty much unambiguous in the context of
virtualisation. 'nested-virt' is only an overstatement of an idiom
that anyone playing with kvmtool will understand.

	M.

-- 
Jazz isn't dead. It just smells funny.