Re: [RFC PATCH 4/5] KVM: arm64: Add KVM_REQ_RELOAD_STAGE2

[Leonardo Bras <leo.bras@arm.com>]

On Wed, Sep 02, 2026 at 11:41:39AM +0800, Tian Zheng wrote:
> 
> 
> On 9/2/2026 1:15 AM, Leonardo Bras wrote:
> > Add a vcpu request to exit guest, reload Stage2, and then come back to
> > guest.
> > 
> > This will be used on future patches that enable S2 HAFDBS and HDBSS, as
> > they may need to change VTCR bits for enabling/disabling the feature when
> > the vcpus are still running.
> > 
> > Signed-off-by: Leonardo Bras <leo.bras@arm.com>
> > ---
> >   arch/arm64/include/asm/kvm_host.h | 2 ++
> >   arch/arm64/kvm/arm.c              | 8 ++++++++
> >   2 files changed, 10 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 27fe0cd5b2d7..00fe169f239f 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -48,20 +48,22 @@
> >   #define KVM_REQ_VCPU_RESET		KVM_ARCH_REQ(2)
> >   #define KVM_REQ_RECORD_STEAL		KVM_ARCH_REQ(3)
> >   #define KVM_REQ_RELOAD_GICv4		KVM_ARCH_REQ(4)
> >   #define KVM_REQ_RELOAD_PMU		KVM_ARCH_REQ(5)
> >   #define KVM_REQ_SUSPEND			KVM_ARCH_REQ(6)
> >   #define KVM_REQ_RESYNC_PMU_EL0		KVM_ARCH_REQ(7)
> >   #define KVM_REQ_NESTED_S2_UNMAP		KVM_ARCH_REQ(8)
> >   #define KVM_REQ_GUEST_HYP_IRQ_PENDING	KVM_ARCH_REQ(9)
> >   #define KVM_REQ_MAP_L1_VNCR_EL2		KVM_ARCH_REQ(10)
> >   #define KVM_REQ_VGIC_PROCESS_UPDATE	KVM_ARCH_REQ(11)
> > +#define KVM_REQ_RELOAD_STAGE2 \
> > +	KVM_ARCH_REQ_FLAGS(12, KVM_REQUEST_WAIT | KVM_REQUEST_NO_WAKEUP)
> >   #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
> >   				     KVM_DIRTY_LOG_INITIALLY_SET)
> >   #define KVM_HAVE_MMU_RWLOCK
> >   /*
> >    * Mode of operation configurable with kvm-arm.mode early param.
> >    * See Documentation/admin-guide/kernel-parameters.txt for more information.
> >    */
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 8b080804bc90..1e528d53d093 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1164,20 +1164,28 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
> >   		if (kvm_check_request(KVM_REQ_RESYNC_PMU_EL0, vcpu))
> >   			kvm_vcpu_pmu_restore_guest(vcpu);
> >   		if (kvm_check_request(KVM_REQ_SUSPEND, vcpu))
> >   			return kvm_vcpu_suspend(vcpu);
> >   		if (kvm_dirty_ring_check_request(vcpu))
> >   			return 0;
> > +		if (kvm_check_request(KVM_REQ_RELOAD_STAGE2, vcpu)) {
> > +			unsigned long flags;
> > +
> > +			local_irq_save(flags);
> > +			__load_stage2(vcpu->arch.hw_mmu);
> > +			local_irq_restore(flags);
> > +		}
> > +
> >   		check_nested_vcpu_requests(vcpu);
> >   	}
> >   	return 1;
> >   }
> >   static bool vcpu_mode_is_bad_32bit(struct kvm_vcpu *vcpu)
> >   {
> >   	if (likely(!vcpu_mode_is_32bit(vcpu)))
> >   		return false;
> Hi Leo,
> 
> Thanks for sending this out. I actually implemented a similar request in
> my v5 HDBSS series to reload VTCR on enable/disable, but I named it
> KVM_REQ_HDBSS_RELOAD_VTCR.
> 
> Without such a request, the new VTCR value only takes effect on the next
> vcpu_load, which means enable/disable could be delayed indefinitely if
> the vCPU never goes through a load/put cycle. So I think this request is
> necessary for HDBSS as well.
> 
> Since your KVM_REQ_RELOAD_STAGE2 is more generic, I'd like to rebase on
> top of your patch and reuse it in v5. Would that work for you? Or do you
> prefer to keep them separate for now?
> 

Hi Tian, 
Thanks for reviewing!

The main reason for this patch series (at least up to patch #3) was to give 
you a base for HDBSS, so please rebase as much as you want on top of it.

Thanks!
Leo