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

[Binbin Wu]

On 4/4/2024 2:34 AM, Isaku Yamahata wrote:
> 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:
> > > +
> > > +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.

To be generic, I think we can use
"vcpu->kvm->arch.hypercall_exit_enabled & (1 << nr)" to check if it 
needs to exit to userspace.

i.e.,

+       ...
+       ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, 
op_64_bit, cpl);
+       if (!ret && (vcpu->kvm->arch.hypercall_exit_enabled & (1 << nr)))
+               /* The hypercall is requested to exit to userspace. */
+               return 0;

> > > +		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