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

Alex Bennée
2015-04-13T07:51:25Z

Andrew Jones <drjones@redhat.com> writes:

> 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
>
> starting comment /* on own line
>
>> + * 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);
>> -
>
> I guess I'll see this come back in the next patch. You must be playing
> 'now you see me, now you don't'

Oops, missed that on the rebase.

>
>> /*
>> * 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);
>> +
>> /*
>> * 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);
>
> With this lines wrapping, {}'s might be nice.

My natural inclination is to wrap in {}'s but I know the kernel is a fan
of the single-statement if forms.

>
>> +
>> + 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;
>
> "old"? As in the old way this worked? Someday (soon) all this code will
> be "old". How about just 'out'? Or use some other way to get the flow
> such that we avoid code duplication, but doesn't require a goto?

I'll see if I can structure this better.

>
>> +
>> + /* 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);
>> + }
>> +
>> return true;
>> }
>>
>> --
>> 2.3.4
>>

--
Alex Bennée
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