Re: [PATCH v2 06/10] KVM: arm64: guest debug, add SW break point support

Alex Bennée
2015-04-02T13:59:14Z

David Hildenbrand <dahi@linux.vnet.ibm.com> writes:

>> This adds support for SW breakpoints inserted by userspace.
>>
>> We do this by trapping all BKPT exceptions in the
>> hypervisor (MDCR_EL2_TDE). The kvm_debug_exit_arch carries the address
>> of the exception. If user-space doesn't know of the breakpoint then we
>> have a guest inserted breakpoint and the hypervisor needs to start again
>> and deliver the exception to guest.
>>
>> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
>>
>> ---
>> v2
>> - update to use new exit struct
>> - tweak for C setup
>> - do our setup in debug_setup/clear code
>> - fixed up comments
>>
>> diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
>> index 06c5064..17d4f9c 100644
>> --- a/Documentation/virtual/kvm/api.txt
>> +++ b/Documentation/virtual/kvm/api.txt
>> @@ -2626,7 +2626,7 @@ when running. Common control bits are:
>> The top 16 bits of the control field are architecture specific control
>> flags which can include the following:
>>
>> - - KVM_GUESTDBG_USE_SW_BP: using software breakpoints [x86]
>> + - KVM_GUESTDBG_USE_SW_BP: using software breakpoints [x86, arm64]
>> - KVM_GUESTDBG_USE_HW_BP: using hardware breakpoints [x86, s390]
>> - KVM_GUESTDBG_INJECT_DB: inject DB type exception [x86]
>> - KVM_GUESTDBG_INJECT_BP: inject BP type exception [x86]
>> diff --git a/arch/arm/kvm/arm.c b/arch/arm/kvm/arm.c
>> index 7ea8b0e..d3bc8dc 100644
>> --- a/arch/arm/kvm/arm.c
>> +++ b/arch/arm/kvm/arm.c
>> @@ -304,7 +304,7 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>> kvm_arm_set_running_vcpu(NULL);
>> }
>>
>> -#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE)
>> +#define KVM_GUESTDBG_VALID (KVM_GUESTDBG_ENABLE|KVM_GUESTDBG_USE_SW_BP)
>>
>> int kvm_arch_vcpu_ioctl_set_guest_debug(struct kvm_vcpu *vcpu,
>> struct kvm_guest_debug *dbg)
>> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
>> index 8a29d0b..cff0475 100644
>> --- a/arch/arm64/kvm/debug.c
>> +++ b/arch/arm64/kvm/debug.c
>> @@ -45,11 +45,18 @@ 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);
>>
>> + /* Trap debug register access? */
>
> This should probably go to the earlier patch.

Agreed.

>
>> if (!vcpu->arch.debug_flags & KVM_ARM64_DEBUG_DIRTY)
>> vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>> else
>> vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDA;
>>
>> + /* Trap breakpoints? */
>> + if (vcpu->guest_debug & KVM_GUESTDBG_USE_SW_BP)
>> + vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
>> + else
>> + vcpu->arch.mdcr_el2 &= ~MDCR_EL2_TDE;
>
> Again, a candidate for clear_debug?

I don't follow. Changes to mdcr_el2 will only get applied as we jump in
through the hyp.S code. We need to ensure the guest can use SW BKPTs if
we are not.

>
>> +
>> }
>>
>> void kvm_arch_clear_debug(struct kvm_vcpu *vcpu)
>> diff --git a/arch/arm64/kvm/handle_exit.c b/arch/arm64/kvm/handle_exit.c
>> index 524fa25..ed1bbb4 100644
>> --- a/arch/arm64/kvm/handle_exit.c
>> +++ b/arch/arm64/kvm/handle_exit.c
>> @@ -82,6 +82,37 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> return 1;
>> }
>>
>> +/**
>> + * kvm_handle_debug_exception - handle a debug exception instruction
>
> "kvm_handle_guest_debug"

Sure.

>
>> + *
>> + * @vcpu: the vcpu pointer
>> + * @run: access to the kvm_run structure for results
>> + *
>> + * We route all debug exceptions through the same handler as we
>> + * just need to report the PC and the HSR values to userspace.
>> + * Userspace may decide to re-inject the exception and deliver it to
>> + * the guest if it wasn't for the host to deal with.
>> + */
>> +static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu, struct kvm_run *run)
>> +{
>> + u32 hsr = kvm_vcpu_get_hsr(vcpu);
>> +
>> + run->exit_reason = KVM_EXIT_DEBUG;
>> + run->debug.arch.hsr = hsr;
>> +
>> + switch (hsr >> ESR_ELx_EC_SHIFT) {
>> + case ESR_ELx_EC_BKPT32:
>> + case ESR_ELx_EC_BRK64:
>> + run->debug.arch.pc = *vcpu_pc(vcpu);
>> + break;
>> + default:
>> + kvm_err("%s: un-handled case hsr: %#08x\n",
>> + __func__, (unsigned int) hsr);
>
> Don't you want to fail hard in this case? This might result in many messages.
> returning 0 feels wrong.

You mean a BUG_ON()? Although it would be a cock up on the hosts part to
have an un-handled exception enabled allowing the guest to trigger a
host panic seems excessive.

>
>> + break;
>> + }
>> + return 0;
>> +}
>> +
>> static exit_handle_fn arm_exit_handlers[] = {
>> [ESR_ELx_EC_WFx] = kvm_handle_wfx,
>> [ESR_ELx_EC_CP15_32] = kvm_handle_cp15_32,
>> @@ -96,6 +127,8 @@ static exit_handle_fn arm_exit_handlers[] = {
>> [ESR_ELx_EC_SYS64] = kvm_handle_sys_reg,
>> [ESR_ELx_EC_IABT_LOW] = kvm_handle_guest_abort,
>> [ESR_ELx_EC_DABT_LOW] = kvm_handle_guest_abort,
>> + [ESR_ELx_EC_BKPT32] = kvm_handle_guest_debug,
>> + [ESR_ELx_EC_BRK64] = kvm_handle_guest_debug,
>> };
>>
>> static exit_handle_fn kvm_get_exit_handler(struct kvm_vcpu *vcpu)
>
>
> David

--
Alex Bennée
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