Re: [PATCH v2 09/10] KVM: arm64: trap nested debug register access

Christoffer Dall
2015-04-14T10:22:15Z

On Tue, Mar 31, 2015 at 04:08:07PM +0100, Alex Bennée wrote:
> When we are using the hardware registers for guest debug we need to deal
> with the guests access to them. There is already a mechanism for dealing
> with these accesses so we build on top of that.
>
> - mdscr_el1_bits is renamed as we save the whole register
> - any access to mdscr_el1 is now stored in the mirror location
> - if we are using HW assisted debug we do the same with DBG[WB][CV]R
>
> There is one register (MDCCINT_EL1) which guest debug doesn't care about
> so this behaves as before.
>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>
> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> index 2c359c9..3d32d45 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -122,10 +122,13 @@ struct kvm_vcpu_arch {
> * here.
> */
>
> - /* Registers pre any guest debug manipulations */
> + /* Registers before any guest debug manipulations. These
> + * shadow registers are updated by the kvm_handle_sys_reg
> + * trap handler if the guest accesses or updates them
> + */
> struct {
> u32 pstate_ss_bit;
> - u32 mdscr_el1_bits;
> + u32 mdscr_el1;
>
> struct kvm_guest_debug_arch debug_regs;
> } debug_saved_regs;
> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
> index 3b368f3..638c111 100644
> --- a/arch/arm64/kvm/debug.c
> +++ b/arch/arm64/kvm/debug.c
> @@ -55,8 +55,6 @@ void kvm_arch_setup_debug(struct kvm_vcpu *vcpu)
> vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM | MDCR_EL2_TPMCR);
> vcpu->arch.mdcr_el2 |= (MDCR_EL2_TDRA | MDCR_EL2_TDOSA);
>
> - trace_kvm_arch_setup_debug_reg32("MDCR_EL2", vcpu->arch.mdcr_el2);
> -
> /*
> * If we are not treating debug registers are dirty we need
> * to trap if the guest starts accessing them.
> @@ -71,8 +69,10 @@ void kvm_arch_setup_debug(struct kvm_vcpu *vcpu)
> /* Save pstate/mdscr */
> vcpu_debug_saved_reg(vcpu, pstate_ss_bit) =
> *vcpu_cpsr(vcpu) & DBG_SPSR_SS;
> - vcpu_debug_saved_reg(vcpu, mdscr_el1_bits) =
> - vcpu_sys_reg(vcpu, MDSCR_EL1) & MDSCR_EL1_DEBUG_BITS;
> +
> + vcpu_debug_saved_reg(vcpu, mdscr_el1) =
> + vcpu_sys_reg(vcpu, MDSCR_EL1);
> +

you can avoid this churn in the patches by following Drew's advice to a
previous patch.

> /*
> * Single Step (ARM ARM D2.12.3 The software step state
> * machine)
> @@ -161,9 +161,8 @@ void kvm_arch_clear_debug(struct kvm_vcpu *vcpu)
> *vcpu_cpsr(vcpu) &= ~DBG_SPSR_SS;
> *vcpu_cpsr(vcpu) |= vcpu_debug_saved_reg(vcpu, pstate_ss_bit);
>
> - vcpu_sys_reg(vcpu, MDSCR_EL1) &= ~MDSCR_EL1_DEBUG_BITS;
> - vcpu_sys_reg(vcpu, MDSCR_EL1) |=
> - vcpu_debug_saved_reg(vcpu, mdscr_el1_bits);
> + vcpu_sys_reg(vcpu, MDSCR_EL1) =
> + vcpu_debug_saved_reg(vcpu, mdscr_el1);
>
> /*
> * If we were using HW debug we need to restore the
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index be9b188..d43d7d1 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -208,39 +208,61 @@ static bool trap_debug_regs(struct kvm_vcpu *vcpu,
> const struct sys_reg_params *p,
> const struct sys_reg_desc *r)
> {
> - if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) {
> - struct kvm_guest_debug_arch *saved;
> - __u64 *val;
> -
> - saved = &vcpu_debug_saved_reg(vcpu, debug_regs);
> -
> - if (r->reg >= DBGBCR0_EL1 && r->reg <= DBGBCR15_EL1)
> - val = &saved->dbg_bcr[r->reg - DBGBCR0_EL1];
> - else if (r->reg >= DBGBVR0_EL1 && r->reg <= DBGBVR15_EL1)
> - val = &saved->dbg_bvr[r->reg - DBGBVR0_EL1];
> - else if (r->reg >= DBGWCR0_EL1 && r->reg <= DBGWCR15_EL1)
> - val = &saved->dbg_wcr[r->reg - DBGWCR0_EL1];
> - else if (r->reg >= DBGWVR0_EL1 && r->reg <= DBGWVR15_EL1)
> - val = &saved->dbg_wvr[r->reg - DBGWVR0_EL1];
> - else {
> - kvm_err("Bad register index %d\n", r->reg);
> - return false;
> + if (vcpu->guest_debug) {
> +
> + /* MDSCR_EL1 */
> + if (r->reg == MDSCR_EL1) {
> + if (p->is_write)
> + vcpu_debug_saved_reg(vcpu, mdscr_el1) =
> + *vcpu_reg(vcpu, p->Rt);
> + else
> + *vcpu_reg(vcpu, p->Rt) =
> + vcpu_debug_saved_reg(vcpu, mdscr_el1);
> +
> + return true;
> }
>
> - if (p->is_write)
> - *val = *vcpu_reg(vcpu, p->Rt);
> - else
> - *vcpu_reg(vcpu, p->Rt) = *val;
> + /* MDCCINT_EL1 */
> + if (r->reg == MDCCINT_EL1)
> + goto old;
> +
> + /* We only shadow DBG* if guest being debugged */
> + if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW_BP) {
> + struct kvm_guest_debug_arch *saved;
> + __u64 *val;
> +
> + saved = &vcpu_debug_saved_reg(vcpu, debug_regs);
> +
> + if (r->reg >= DBGBCR0_EL1 && r->reg <= DBGBCR15_EL1)
> + val = &saved->dbg_bcr[r->reg - DBGBCR0_EL1];
> + else if (r->reg >= DBGBVR0_EL1 && r->reg <= DBGBVR15_EL1)
> + val = &saved->dbg_bvr[r->reg - DBGBVR0_EL1];
> + else if (r->reg >= DBGWCR0_EL1 && r->reg <= DBGWCR15_EL1)
> + val = &saved->dbg_wcr[r->reg - DBGWCR0_EL1];
> + else if (r->reg >= DBGWVR0_EL1 && r->reg <= DBGWVR15_EL1)
> + val = &saved->dbg_wvr[r->reg - DBGWVR0_EL1];
> + else {
> + kvm_err("Bad register index %d\n", r->reg);
> + return false;
> + }
>
> - } else {
> - if (p->is_write) {
> - vcpu_sys_reg(vcpu, r->reg) = *vcpu_reg(vcpu, p->Rt);
> - vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> - } else {
> - *vcpu_reg(vcpu, p->Rt) = vcpu_sys_reg(vcpu, r->reg);
> + if (p->is_write)
> + *val = *vcpu_reg(vcpu, p->Rt);
> + else
> + *vcpu_reg(vcpu, p->Rt) = *val;
> +
> + return true;
> }
> }
>
> +old:
> + if (p->is_write) {
> + vcpu_sys_reg(vcpu, r->reg) = *vcpu_reg(vcpu, p->Rt);
> + vcpu->arch.debug_flags |= KVM_ARM64_DEBUG_DIRTY;
> + } else {
> + *vcpu_reg(vcpu, p->Rt) = vcpu_sys_reg(vcpu, r->reg);
> + }
> +

I really think this points to a problem with the design; the emulate
function should just emulate writes/reads to some state without this
complexity. If there's some reason not to do this, you should put that
in the commit text.

> return true;
> }
>
> --
> 2.3.4
>

Thanks,
-Christoffer