Re: [PATCH v2 2/4] KVM: arm64: sefltests: Add helpers for guest hypervisors

[Wei-Lin Chang]

On Wed, Apr 15, 2026 at 07:14:46AM +0900, Itaru Kitayama wrote:
> On Sun, Apr 12, 2026 at 03:22:14PM +0100, Wei-Lin Chang wrote:
> > Add helpers so that guest hypervisors can run nested guests. SP_EL1
> > save/restore is added to allow nested guests to use a stack.
> > 
> > Signed-off-by: Wei-Lin Chang <weilin.chang@xxxxxxx>
> > ---
> >  .../selftests/kvm/include/arm64/nested.h      | 17 +++++++
> >  tools/testing/selftests/kvm/lib/arm64/entry.S |  5 ++
> >  .../testing/selftests/kvm/lib/arm64/nested.c  | 46 +++++++++++++++++++
> >  3 files changed, 68 insertions(+)
> > 
> > diff --git a/tools/testing/selftests/kvm/include/arm64/nested.h b/tools/testing/selftests/kvm/include/arm64/nested.h
> > index 86d931facacb..7928ef89494a 100644
> > --- a/tools/testing/selftests/kvm/include/arm64/nested.h
> > +++ b/tools/testing/selftests/kvm/include/arm64/nested.h
> > @@ -21,8 +21,17 @@
> >  
> >  extern char hyp_vectors[];
> >  
> > +enum vcpu_sysreg {
> > +	__INVALID_SYSREG__,   /* 0 is reserved as an invalid value */
> > +
> > +	SP_EL1,
> > +
> > +	NR_SYS_REGS
> > +};
> > +
> >  struct cpu_context {
> >  	struct user_pt_regs regs;	/* sp = sp_el0 */
> > +	u64 sys_regs[NR_SYS_REGS];
> >  };
> >  
> >  struct vcpu {
> > @@ -37,9 +46,17 @@ struct hyp_data {
> >  	struct cpu_context hyp_context;
> >  };
> 
> I am not sure of these structs you introduced only for nested guest feature
> testing, as the KVM arm64 code they are quite complex and involved, 
> extracring part of those and add members as hello_nested or simliar
> tests evolve, then add test cases to me seems fragile. 
> But if you have strong reason to add these would you mind explaining a bit?

Sorry, I don't quite get all of your points. I understand your argument
being evolving these structs as time goes is fragile. For this didn't
KVM itself evolve like this?

As for having these structs, how can we make L1 a small hypervisor
without them?

Thanks,
Wei-Lin Chang

> 
> Thanks,
> Itaru.
> 
> >  
> > +void prepare_hyp(void);
> > +void init_vcpu(struct vcpu *vcpu, vm_paddr_t l2_pc, vm_paddr_t l2_stack_top);
> > +int run_l2(struct vcpu *vcpu, struct hyp_data *hyp_data);
> > +
> > +void do_hvc(void);
> >  u64 __guest_enter(struct vcpu *vcpu, struct cpu_context *hyp_context);
> >  void __hyp_exception(u64 type);
> >  
> > +void __sysreg_save_el1_state(struct cpu_context *ctxt);
> > +void __sysreg_restore_el1_state(struct cpu_context *ctxt);
> > +
> >  #endif /* !__ASSEMBLER__ */
> >  
> >  #endif /* SELFTEST_KVM_NESTED_H */
> > diff --git a/tools/testing/selftests/kvm/lib/arm64/entry.S b/tools/testing/selftests/kvm/lib/arm64/entry.S
> > index 33bedf5e7fb2..df3af3463c6c 100644
> > --- a/tools/testing/selftests/kvm/lib/arm64/entry.S
> > +++ b/tools/testing/selftests/kvm/lib/arm64/entry.S
> > @@ -3,6 +3,11 @@
> >   * adapted from arch/arm64/kvm/hyp/entry.S
> >   */
> >  
> > + .globl do_hvc
> > + do_hvc:
> > +	hvc	#0
> > +	ret
> > +
> >  /*
> >   * Manually define these for now
> >   */
> > diff --git a/tools/testing/selftests/kvm/lib/arm64/nested.c b/tools/testing/selftests/kvm/lib/arm64/nested.c
> > index 06ddaab2436f..b30d20b101c4 100644
> > --- a/tools/testing/selftests/kvm/lib/arm64/nested.c
> > +++ b/tools/testing/selftests/kvm/lib/arm64/nested.c
> > @@ -4,7 +4,53 @@
> >   */
> >  
> >  #include "nested.h"
> > +#include "processor.h"
> >  #include "test_util.h"
> > +#include <asm/sysreg.h>
> > +
> > +void prepare_hyp(void)
> > +{
> > +	write_sysreg(HCR_EL2_E2H | HCR_EL2_RW, hcr_el2);
> > +	write_sysreg(hyp_vectors, vbar_el2);
> > +	isb();
> > +}
> > +
> > +void init_vcpu(struct vcpu *vcpu, vm_paddr_t l2_pc, vm_paddr_t l2_stack_top)
> > +{
> > +	memset(vcpu, 0, sizeof(*vcpu));
> > +	vcpu->context.regs.pc = l2_pc;
> > +	vcpu->context.regs.pstate = PSR_MODE_EL1h | PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT;
> > +	vcpu->context.sys_regs[SP_EL1] = l2_stack_top;
> > +}
> > +
> > +void __sysreg_save_el1_state(struct cpu_context *ctxt)
> > +{
> > +	ctxt->sys_regs[SP_EL1] = read_sysreg(sp_el1);
> > +}
> > +
> > +void __sysreg_restore_el1_state(struct cpu_context *ctxt)
> > +{
> > +	write_sysreg(ctxt->sys_regs[SP_EL1], sp_el1);
> > +}
> > +
> > +int run_l2(struct vcpu *vcpu, struct hyp_data *hyp_data)
> > +{
> > +	u64 ret;
> > +
> > +	__sysreg_restore_el1_state(&vcpu->context);
> > +
> > +	write_sysreg(vcpu->context.regs.pstate, spsr_el2);
> > +	write_sysreg(vcpu->context.regs.pc, elr_el2);
> > +
> > +	ret =  __guest_enter(vcpu, &hyp_data->hyp_context);
> > +
> > +	vcpu->context.regs.pc = read_sysreg(elr_el2);
> > +	vcpu->context.regs.pstate = read_sysreg(spsr_el2);
> > +
> > +	__sysreg_save_el1_state(&vcpu->context);
> > +
> > +	return ret;
> > +}
> >  
> >  void __hyp_exception(u64 type)
> >  {
> > -- 
> > 2.43.0
> >