[PATCH v2] KVM: arm64: Initialize VCPU mdcr_el2 before loading it
Alexandru Elisei
alexandru.elisei at arm.com
Tue Mar 30 18:13:07 BST 2021
Hi Marc,
Thanks for having a look!
On 3/30/21 10:55 AM, Marc Zyngier wrote:
> Hi Alex,
>
> On Tue, 23 Mar 2021 18:00:57 +0000,
> Alexandru Elisei <alexandru.elisei at arm.com> wrote:
>> When a VCPU is created, the kvm_vcpu struct is initialized to zero in
>> kvm_vm_ioctl_create_vcpu(). On VHE systems, the first time
>> vcpu.arch.mdcr_el2 is loaded on hardware is in vcpu_load(), before it is
>> set to a sensible value in kvm_arm_setup_debug() later in the run loop. The
>> result is that KVM executes for a short time with MDCR_EL2 set to zero.
>>
>> This has several unintended consequences:
>>
>> * Setting MDCR_EL2.HPMN to 0 is constrained unpredictable according to ARM
>> DDI 0487G.a, page D13-3820. The behavior specified by the architecture
>> in this case is for the PE to behave as if MDCR_EL2.HPMN is set to a
>> value less than or equal to PMCR_EL0.N, which means that an unknown
>> number of counters are now disabled by MDCR_EL2.HPME, which is zero.
>>
>> * The host configuration for the other debug features controlled by
>> MDCR_EL2 is temporarily lost. This has been harmless so far, as Linux
>> doesn't use the other fields, but that might change in the future.
>>
>> Let's avoid both issues by initializing the VCPU's mdcr_el2 field in
>> kvm_vcpu_vcpu_first_run_init(), thus making sure that the MDCR_EL2 register
>> has a consistent value after each vcpu_load().
>>
>> Signed-off-by: Alexandru Elisei <alexandru.elisei at arm.com>
> This looks strangely similar to 4942dc6638b0 ("KVM: arm64: Write
> arch.mdcr_el2 changes since last vcpu_load on VHE"), just at a
> different point. Probably worth a Fixes tag.
This bug is present in the commit you are mentioning, and from what I can tell
it's also present in the commit it's fixing (d5a21bcc2995 ("KVM: arm64: Move
common VHE/non-VHE trap config in separate functions")) - vcpu->arch.mdcr_el2 is
computed in kvm_arm_setup_debug(), which is called after vcpu_load(). My guess is
that this bug is from VHE support was added (or soon after).
I can dig further, how far back in time should I aim for?
>
>> ---
>> Found by code inspection. Based on v5.12-rc4.
>>
>> Tested on an odroid-c4 with VHE. vcpu->arch.mdcr_el2 is calculated to be
>> 0x4e66. Without this patch, reading MDCR_EL2 after the first vcpu_load() in
>> kvm_arch_vcpu_ioctl_run() returns 0; with this patch it returns the correct
>> value, 0xe66 (FEAT_SPE is not implemented by the PE).
>>
>> This patch was initially part of the KVM SPE series [1], but those patches
>> haven't seen much activity, so I thought it would be a good idea to send
>> this patch separately to draw more attention to it.
>>
>> Changes in v2:
>> * Moved kvm_arm_vcpu_init_debug() earlier in kvm_vcpu_first_run_init() so
>> vcpu->arch.mdcr_el2 is calculated even if kvm_vgic_map_resources() fails.
>> * Added comment to kvm_arm_setup_mdcr_el2 to explain what testing
>> vcpu->guest_debug means.
>>
>> [1] https://www.spinics.net/lists/kvm-arm/msg42959.html
>>
>> arch/arm64/include/asm/kvm_host.h | 1 +
>> arch/arm64/kvm/arm.c | 3 +-
>> arch/arm64/kvm/debug.c | 82 +++++++++++++++++++++----------
>> 3 files changed, 59 insertions(+), 27 deletions(-)
>>
>> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
>> index 3d10e6527f7d..858c2fcfc043 100644
>> --- a/arch/arm64/include/asm/kvm_host.h
>> +++ b/arch/arm64/include/asm/kvm_host.h
>> @@ -713,6 +713,7 @@ static inline void kvm_arch_sched_in(struct kvm_vcpu *vcpu, int cpu) {}
>> static inline void kvm_arch_vcpu_block_finish(struct kvm_vcpu *vcpu) {}
>>
>> void kvm_arm_init_debug(void);
>> +void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu);
>> void kvm_arm_setup_debug(struct kvm_vcpu *vcpu);
>> void kvm_arm_clear_debug(struct kvm_vcpu *vcpu);
>> void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu);
>> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
>> index 7f06ba76698d..7088d8fe7186 100644
>> --- a/arch/arm64/kvm/arm.c
>> +++ b/arch/arm64/kvm/arm.c
>> @@ -580,6 +580,8 @@ static int kvm_vcpu_first_run_init(struct kvm_vcpu *vcpu)
>>
>> vcpu->arch.has_run_once = true;
>>
>> + kvm_arm_vcpu_init_debug(vcpu);
>> +
>> if (likely(irqchip_in_kernel(kvm))) {
>> /*
>> * Map the VGIC hardware resources before running a vcpu the
>> @@ -791,7 +793,6 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu)
>> }
>>
>> kvm_arm_setup_debug(vcpu);
>> -
> Spurious change?
Definitely, thank you for spotting it.
>
>> /**************************************************************
>> * Enter the guest
>> */
>> diff --git a/arch/arm64/kvm/debug.c b/arch/arm64/kvm/debug.c
>> index 7a7e425616b5..3626d03354f6 100644
>> --- a/arch/arm64/kvm/debug.c
>> +++ b/arch/arm64/kvm/debug.c
>> @@ -68,6 +68,60 @@ void kvm_arm_init_debug(void)
>> __this_cpu_write(mdcr_el2, kvm_call_hyp_ret(__kvm_get_mdcr_el2));
>> }
>>
>> +/**
>> + * kvm_arm_setup_mdcr_el2 - configure vcpu mdcr_el2 value
>> + *
>> + * @vcpu: the vcpu pointer
>> + * @host_mdcr: host mdcr_el2 value
>> + *
>> + * This ensures we will trap access to:
>> + * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
>> + * - Debug ROM Address (MDCR_EL2_TDRA)
>> + * - OS related registers (MDCR_EL2_TDOSA)
>> + * - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
>> + */
>> +static void kvm_arm_setup_mdcr_el2(struct kvm_vcpu *vcpu, u32 host_mdcr)
>> +{
>> + bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
>> +
>> + /*
>> + * This also clears MDCR_EL2_E2PB_MASK to disable guest access
>> + * to the profiling buffer.
>> + */
>> + vcpu->arch.mdcr_el2 = host_mdcr & MDCR_EL2_HPMN_MASK;
>> + vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
>> + MDCR_EL2_TPMS |
>> + MDCR_EL2_TPMCR |
>> + MDCR_EL2_TDRA |
>> + MDCR_EL2_TDOSA);
>> +
>> + /* Is the VM being debugged by userspace? */
>> + if (vcpu->guest_debug) {
>> + /* Route all software debug exceptions to EL2 */
>> + vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
>> + if (vcpu->guest_debug & KVM_GUESTDBG_USE_HW)
>> + trap_debug = true;
>> + }
>> +
>> + /* Trap debug register access */
>> + if (trap_debug)
>> + vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>> +
>> + trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
>> +}
>> +
>> +/**
>> + * kvm_arm_vcpu_init_debug - setup vcpu debug traps
>> + *
>> + * @vcpu: the vcpu pointer
>> + *
>> + * Set vcpu initial mdcr_el2 value.
>> + */
>> +void kvm_arm_vcpu_init_debug(struct kvm_vcpu *vcpu)
>> +{
>> + kvm_arm_setup_mdcr_el2(vcpu, this_cpu_read(mdcr_el2));
> Given that kvm_arm_setup_mdcr_el2() always takes the current host
> value for mdcr_el2, why not moving the read into it and be done with
> it?
kvm_arm_setup_debug() is called with preemption disabled, and it can use
__this_cpu_read(). kvm_arm_vcpu_init_debug() is called with preemption enabled, so
it must use this_cpu_read(). I wanted to make the distinction because
kvm_arm_setup_debug() is in the run loop.
>
> Also, do we really need an extra wrapper?
I can remove the wrapper and have kvm_arm_setup_mdcr_el2() use this_cpu_read() for
the host's mdcr_el2 value at the cost of a preempt disable/enable in the run loop
when preemption is disabled. If you think that would make the code easier to
follow, I can certainly do that.
>
>> +}
>> +
>> /**
>> * kvm_arm_reset_debug_ptr - reset the debug ptr to point to the vcpu state
>> */
>> @@ -83,12 +137,7 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>> * @vcpu: the vcpu pointer
>> *
>> * This is called before each entry into the hypervisor to setup any
>> - * debug related registers. Currently this just ensures we will trap
>> - * access to:
>> - * - Performance monitors (MDCR_EL2_TPM/MDCR_EL2_TPMCR)
>> - * - Debug ROM Address (MDCR_EL2_TDRA)
>> - * - OS related registers (MDCR_EL2_TDOSA)
>> - * - Statistical profiler (MDCR_EL2_TPMS/MDCR_EL2_E2PB)
>> + * debug related registers.
>> *
>> * Additionally, KVM only traps guest accesses to the debug registers if
>> * the guest is not actively using them (see the KVM_ARM64_DEBUG_DIRTY
>> @@ -100,27 +149,14 @@ void kvm_arm_reset_debug_ptr(struct kvm_vcpu *vcpu)
>>
>> void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>> {
>> - bool trap_debug = !(vcpu->arch.flags & KVM_ARM64_DEBUG_DIRTY);
>> unsigned long mdscr, orig_mdcr_el2 = vcpu->arch.mdcr_el2;
>>
>> trace_kvm_arm_setup_debug(vcpu, vcpu->guest_debug);
>>
>> - /*
>> - * This also clears MDCR_EL2_E2PB_MASK to disable guest access
>> - * to the profiling buffer.
>> - */
>> - vcpu->arch.mdcr_el2 = __this_cpu_read(mdcr_el2) & MDCR_EL2_HPMN_MASK;
>> - vcpu->arch.mdcr_el2 |= (MDCR_EL2_TPM |
>> - MDCR_EL2_TPMS |
>> - MDCR_EL2_TPMCR |
>> - MDCR_EL2_TDRA |
>> - MDCR_EL2_TDOSA);
>> + kvm_arm_setup_mdcr_el2(vcpu, __this_cpu_read(mdcr_el2));
>>
>> /* Is Guest debugging in effect? */
>> if (vcpu->guest_debug) {
>> - /* Route all software debug exceptions to EL2 */
>> - vcpu->arch.mdcr_el2 |= MDCR_EL2_TDE;
>> -
>> /* Save guest debug state */
>> save_guest_debug_regs(vcpu);
>>
>> @@ -174,7 +210,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>>
>> vcpu->arch.debug_ptr = &vcpu->arch.external_debug_state;
>> vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
>> - trap_debug = true;
> There is something that slightly worries me here: there is now a
> disconnect between flagging debug as dirty and setting the
> trapping. And actually, you now check for KVM_ARM64_DEBUG_DIRTY and
> set the trap bits *before* setting the dirty bit itself.
>
> Here, I believe you end up with guest/host confusion of breakpoints,
> which isn't great. Or did I miss something?
I'm sorry, but I don't understand what you mean. This is my understanding of what
is happening.
Without this patch, trap_debug is set to true and the KVM_ARM64_DEBUG_DIRTY flag
is set if vcpu->guest_debug & KVM_GUESTDBG_USE_HW. Further down, trap debug is
only used when computing mdcr_el2.
With this patch, trap_debug is set to true if vcpu->guest_debug &
KVM_GUESTDBG_USE_HW and it's also used for computing mdcr_el2, but this happens in
kvm_arm_setup_mdcr_el2(), which is called at the start of kvm_arm_setup_debug().
The KVM_ARM_DEBUG_DIRTY flags is still set in kvm_arm_setup_debug() if
vcpu->guest_debug & KVM_GUESTDBG_USE_HW, like before.
The guest never runs with the value computed in kvm_vcpu_first_run_init() unless
it's identical with the value recomputed in kvm_arm_setup_debug().
The only difference I see is that mdcr_el2 is computed at the start of
kvm_arm_setup_debug(). I get the feeling I'm also missing something.
Thanks,
Alex
>
>>
>> trace_kvm_arm_set_regset("BKPTS", get_num_brps(),
>> &vcpu->arch.debug_ptr->dbg_bcr[0],
>> @@ -189,10 +224,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>> BUG_ON(!vcpu->guest_debug &&
>> vcpu->arch.debug_ptr != &vcpu->arch.vcpu_debug_state);
>>
>> - /* Trap debug register access */
>> - if (trap_debug)
>> - vcpu->arch.mdcr_el2 |= MDCR_EL2_TDA;
>> -
>> /* If KDE or MDE are set, perform a full save/restore cycle. */
>> if (vcpu_read_sys_reg(vcpu, MDSCR_EL1) & (DBG_MDSCR_KDE | DBG_MDSCR_MDE))
>> vcpu->arch.flags |= KVM_ARM64_DEBUG_DIRTY;
>> @@ -201,7 +232,6 @@ void kvm_arm_setup_debug(struct kvm_vcpu *vcpu)
>> if (has_vhe() && orig_mdcr_el2 != vcpu->arch.mdcr_el2)
>> write_sysreg(vcpu->arch.mdcr_el2, mdcr_el2);
>>
>> - trace_kvm_arm_set_dreg32("MDCR_EL2", vcpu->arch.mdcr_el2);
>> trace_kvm_arm_set_dreg32("MDSCR_EL1", vcpu_read_sys_reg(vcpu, MDSCR_EL1));
>> }
> Thanks,
>
> M.
>
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