Re: [PATCH v19 097/130] KVM: x86: Split core of hypercall emulation to helper function

[Isaku Yamahata]

On Fri, Mar 29, 2024 at 11:24:55AM +0800,
Chao Gao <chao.gao@xxxxxxxxx> wrote:

> On Mon, Feb 26, 2024 at 12:26:39AM -0800, isaku.yamahata@xxxxxxxxx wrote:
> >@@ -10162,18 +10151,49 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> > 
> > 		WARN_ON_ONCE(vcpu->run->hypercall.flags & KVM_EXIT_HYPERCALL_MBZ);
> > 		vcpu->arch.complete_userspace_io = complete_hypercall_exit;
> >+		/* stat is incremented on completion. */
> 
> Perhaps we could use a distinct return value to signal that the request is redirected
> to userspace. This way, more cases can be supported, e.g., accesses to MTRR
> MSRs, requests to service TDs, etc. And then ...

The convention here is the one for exit_handler vcpu_enter_guest() already uses.
If we introduce something like KVM_VCPU_CONTINUE=1, KVM_VCPU_EXIT_TO_USER=0, it
will touch many places.  So if we will (I'm not sure it's worthwhile), the
cleanup should be done as independently.


> > 		return 0;
> > 	}
> > 	default:
> > 		ret = -KVM_ENOSYS;
> > 		break;
> > 	}
> >+
> > out:
> >+	++vcpu->stat.hypercalls;
> >+	return ret;
> >+}
> >+EXPORT_SYMBOL_GPL(__kvm_emulate_hypercall);
> >+
> >+int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> >+{
> >+	unsigned long nr, a0, a1, a2, a3, ret;
> >+	int op_64_bit;
> >+	int cpl;
> >+
> >+	if (kvm_xen_hypercall_enabled(vcpu->kvm))
> >+		return kvm_xen_hypercall(vcpu);
> >+
> >+	if (kvm_hv_hypercall_enabled(vcpu))
> >+		return kvm_hv_hypercall(vcpu);
> >+
> >+	nr = kvm_rax_read(vcpu);
> >+	a0 = kvm_rbx_read(vcpu);
> >+	a1 = kvm_rcx_read(vcpu);
> >+	a2 = kvm_rdx_read(vcpu);
> >+	a3 = kvm_rsi_read(vcpu);
> >+	op_64_bit = is_64_bit_hypercall(vcpu);
> >+	cpl = static_call(kvm_x86_get_cpl)(vcpu);
> >+
> >+	ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
> >+	if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
> >+		/* MAP_GPA tosses the request to the user space. */
> 
> no need to check what the request is. Just checking the return value will suffice.

This is needed to avoid updating rax etc.  KVM_HC_MAP_GPA_RANGE is only an
exception to go to the user space.  This check is a bit weird, but I couldn't
find a good way.

> 
> >+		return 0;
> >+
> > 	if (!op_64_bit)
> > 		ret = (u32)ret;
> > 	kvm_rax_write(vcpu, ret);
> > 
> >-	++vcpu->stat.hypercalls;
> > 	return kvm_skip_emulated_instruction(vcpu);
> > }
> > EXPORT_SYMBOL_GPL(kvm_emulate_hypercall);
> >-- 
> >2.25.1
> >
> >
> 
-- 
Isaku Yamahata <isaku.yamahata@xxxxxxxxx>