Re: [PATCH v2 6/9] arm64: KVM: Treat sysreg accessors returning false as successful

Christoffer Dall
2017-03-28T12:43:34Z

On Mon, Mar 27, 2017 at 05:03:42PM +0100, Marc Zyngier wrote:
> Instead of considering that a sysreg accessor has failed when
> returning false, let's consider that it is *always* successful
> (after all, we won't stand for an incomplete emulation).

That's right!

>
> The return value now simply indicates whether we should skip
> the instruction (because it has now been emulated), or if we
> should leave the PC alone if the emulation has injected an
> exception.

Reviewed-by: Christoffer Dall <cdall@linaro.org>

(I especially enjoy the much cleaner flow of emulate_sys_reg())

>
> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> ---
> arch/arm64/kvm/sys_regs.c | 49 +++++++++++++++++++----------------------------
> 1 file changed, 20 insertions(+), 29 deletions(-)
>
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index f80a61af5e88..4e5d4eee8cec 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1571,6 +1571,22 @@ int kvm_handle_cp14_load_store(struct kvm_vcpu *vcpu, struct kvm_run *run)
> return 1;
> }
>
> +static void perform_access(struct kvm_vcpu *vcpu,
> + struct sys_reg_params *params,
> + const struct sys_reg_desc *r)
> +{
> + /*
> + * Not having an accessor means that we have configured a trap
> + * that we don't know how to handle. This certainly qualifies
> + * as a gross bug that should be fixed right away.
> + */
> + BUG_ON(!r->access);
> +
> + /* Skip instruction if instructed so */
> + if (likely(r->access(vcpu, params, r)))
> + kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> +}
> +
> /*
> * emulate_cp -- tries to match a sys_reg access in a handling table, and
> * call the corresponding trap handler.
> @@ -1594,20 +1610,8 @@ static int emulate_cp(struct kvm_vcpu *vcpu,
> r = find_reg(params, table, num);
>
> if (r) {
> - /*
> - * Not having an accessor means that we have
> - * configured a trap that we don't know how to
> - * handle. This certainly qualifies as a gross bug
> - * that should be fixed right away.
> - */
> - BUG_ON(!r->access);
> -
> - if (likely(r->access(vcpu, params, r))) {
> - /* Skip instruction, since it was emulated */
> - kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> - /* Handled */
> - return 0;
> - }
> + perform_access(vcpu, params, r);
> + return 0;
> }
>
> /* Not handled */
> @@ -1777,26 +1781,13 @@ static int emulate_sys_reg(struct kvm_vcpu *vcpu,
> r = find_reg(params, sys_reg_descs, ARRAY_SIZE(sys_reg_descs));
>
> if (likely(r)) {
> - /*
> - * Not having an accessor means that we have
> - * configured a trap that we don't know how to
> - * handle. This certainly qualifies as a gross bug
> - * that should be fixed right away.
> - */
> - BUG_ON(!r->access);
> -
> - if (likely(r->access(vcpu, params, r))) {
> - /* Skip instruction, since it was emulated */
> - kvm_skip_instr(vcpu, kvm_vcpu_trap_il_is32bit(vcpu));
> - return 1;
> - }
> - /* If access function fails, it should complain. */
> + perform_access(vcpu, params, r);
> } else {
> kvm_err("Unsupported guest sys_reg access at: %lx\n",
> *vcpu_pc(vcpu));
> print_sys_reg_instr(params);
> + kvm_inject_undefined(vcpu);
> }
> - kvm_inject_undefined(vcpu);
> return 1;
> }
>
> --
> 2.11.0
>