On Sat, Nov 28, 2020, David Woodhouse wrote:
...
> +static int complete_userspace_hypercall(struct kvm_vcpu *vcpu)
> +{
> + kvm_rax_write(vcpu, vcpu->run->hypercall.ret);
> +
> + return kvm_skip_emulated_instruction(vcpu);
This should probably verify the linear RIP is unchanged before modifying guest
state, e.g. to let userspace reset the vCPU in response to a hypercall. See
complete_fast_pio_out().
> +}
> +
> +int kvm_userspace_hypercall(struct kvm_vcpu *vcpu)
> +{
> + struct kvm_run *run = vcpu->run;
> +
> + if (is_long_mode(vcpu)) {
> + run->hypercall.longmode = 1;
Should this also capture the CPL? I assume the motivation for grabbing regs
and longmode is to avoid having to call back into KVM on every hypercall, and I
also assume there are (or will be) hypercalls that are CPL0 only.
> + run->hypercall.nr = kvm_rax_read(vcpu);
> + run->hypercall.args[0] = kvm_rdi_read(vcpu);
> + run->hypercall.args[1] = kvm_rsi_read(vcpu);
> + run->hypercall.args[2] = kvm_rdx_read(vcpu);
> + run->hypercall.args[3] = kvm_r10_read(vcpu);
> + run->hypercall.args[4] = kvm_r8_read(vcpu);
> + run->hypercall.args[5] = kvm_r9_read(vcpu);
> + run->hypercall.ret = -KVM_ENOSYS;
> + } else {
> + run->hypercall.longmode = 0;
> + run->hypercall.nr = (u32)kvm_rbx_read(vcpu);
> + run->hypercall.args[0] = (u32)kvm_rbx_read(vcpu);
> + run->hypercall.args[1] = (u32)kvm_rcx_read(vcpu);
> + run->hypercall.args[2] = (u32)kvm_rdx_read(vcpu);
> + run->hypercall.args[3] = (u32)kvm_rsi_read(vcpu);
> + run->hypercall.args[4] = (u32)kvm_rdi_read(vcpu);
> + run->hypercall.args[5] = (u32)kvm_rbp_read(vcpu);
> + run->hypercall.ret = (u32)-KVM_ENOSYS;
> + }
Why not get/set all GPRs (except maybe RIP and RSP) as opposed to implementing
what I presume is Xen's ABI in a generic KVM user exit? Copying 10 more GPRs
to/from memory is likely lost in the noise relative to the cost of the userspace
roundtrip.
> + run->exit_reason = KVM_EXIT_HYPERCALL;
> + vcpu->arch.complete_userspace_io = complete_userspace_hypercall;
> +
> + return 0;
> +}
> +
> int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
> {
> unsigned long nr, a0, a1, a2, a3, ret;
> int op_64_bit;
>
> + if (vcpu->kvm->arch.user_space_hypercall)
> + return kvm_userspace_hypercall(vcpu);
> +
> if (kvm_hv_hypercall_enabled(vcpu->kvm))
> return kvm_hv_hypercall(vcpu);
>
