Re: [PATCH v2 02/10] KVM: PPC: Book3S HV: Save/restore new PMU registers

On 01-Jul-2020, at 4:41 PM, Paul Mackerras [off-list ref] wrote:

On Wed, Jul 01, 2020 at 05:20:54AM -0400, Athira Rajeev wrote:

quoted

PowerISA v3.1 has added new performance monitoring unit (PMU)
special purpose registers (SPRs). They are

Monitor Mode Control Register 3 (MMCR3)
Sampled Instruction Event Register A (SIER2)
Sampled Instruction Event Register B (SIER3)

Patch addes support to save/restore these new
SPRs while entering/exiting guest.

This mostly looks reasonable, at a quick glance at least, but I am
puzzled by two of the changes you are making.  See below.

quoted

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 6bf66649..c265800 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -1698,7 +1698,8 @@ static int kvmppc_get_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,

		*val = get_reg_val(id, vcpu->arch.sdar);
		break;
	case KVM_REG_PPC_SIER:
-		*val = get_reg_val(id, vcpu->arch.sier);
+		i = id - KVM_REG_PPC_SIER;
+		*val = get_reg_val(id, vcpu->arch.sier[i]);

This is inside a switch (id) statement, so here we know that id is
KVM_REG_PPC_SIER.  In other words i will always be zero, so what is
the point of doing the subtraction?

quoted

		break;
	case KVM_REG_PPC_IAMR:
		*val = get_reg_val(id, vcpu->arch.iamr);

@@ -1919,7 +1920,8 @@ static int kvmppc_set_one_reg_hv(struct kvm_vcpu *vcpu, u64 id,

		vcpu->arch.sdar = set_reg_val(id, *val);
		break;
	case KVM_REG_PPC_SIER:
-		vcpu->arch.sier = set_reg_val(id, *val);
+		i = id - KVM_REG_PPC_SIER;
+		vcpu->arch.sier[i] = set_reg_val(id, *val);

Same comment here.

Hi Paul,

Thanks for reviewing the patch. Yes, true that currently `id` will be zero since it is only KVM_REG_PPC_SIER. I have kept the subtraction here considering that there will be addition of new registers to switch case. 
ex: case KVM_REG_PPC_SIER..KVM_REG_PPC_SIER3

I think that new defines for the new registers will need to be added
to arch/powerpc/include/uapi/asm/kvm.h and
Documentation/virt/kvm/api.rst, and then new cases will need to be
added to these switch statements.

Yes, New registers are not yet added to kvm.h 
I will address these comments and include changes for arch/powerpc/include/uapi/asm/kvm.h and Documentation/virt/kvm/api.rst in the
next version.

By the way, please cc kvm-ppc@vger.kernel.org and kvm@vger.kernel.org
on KVM patches.

Sure, will include KVM mailing list in the next version

Thanks
Athira 

Paul.