On 09/09/2019 13:13, Christoffer Dall wrote:
> For a long time, if a guest accessed memory outside of a memslot using
> any of the load/store instructions in the architecture which doesn't
> supply decoding information in the ESR_EL2 (the ISV bit is not set), the
> kernel would print the following message and terminate the VM as a
> result of returning -ENOSYS to userspace:
>
> load/store instruction decoding not implemented
>
> The reason behind this message is that KVM assumes that all accesses
> outside a memslot is an MMIO access which should be handled by
> userspace, and we originally expected to eventually implement some sort
> of decoding of load/store instructions where the ISV bit was not set.
>
> However, it turns out that many of the instructions which don't provide
> decoding information on abort are not safe to use for MMIO accesses, and
> the remaining few that would potentially make sense to use on MMIO
> accesses, such as those with register writeback, are not used in
> practice. It also turns out that fetching an instruction from guest
> memory can be a pretty horrible affair, involving stopping all CPUs on
> SMP systems, handling multiple corner cases of address translation in
> software, and more. It doesn't appear likely that we'll ever implement
> this in the kernel.
>
> What is much more common is that a user has misconfigured his/her guest
> and is actually not accessing an MMIO region, but just hitting some
> random hole in the IPA space. In this scenario, the error message above
> is almost misleading and has led to a great deal of confusion over the
> years.
>
> It is, nevertheless, ABI to userspace, and we therefore need to
> introduce a new capability that userspace explicitly enables to change
> behavior.
>
> This patch introduces KVM_CAP_ARM_NISV_TO_USER (NISV meaning Non-ISV)
> which does exactly that, and introduces a new exit reason to report the
> event to userspace. User space can then emulate an exception to the
> guest, restart the guest, suspend the guest, or take any other
> appropriate action as per the policy of the running system.
>
> Reported-by: Heinrich Schuchardt <[email protected]>
> Signed-off-by: Christoffer Dall <[email protected]>
> ---
> Documentation/virt/kvm/api.txt | 29 ++++++++++++++++++++++++++++
> arch/arm/include/asm/kvm_arm.h | 2 ++
> arch/arm/include/asm/kvm_emulate.h | 5 +++++
> arch/arm/include/asm/kvm_host.h | 8 ++++++++
> arch/arm64/include/asm/kvm_emulate.h | 5 +++++
> arch/arm64/include/asm/kvm_host.h | 8 ++++++++
> include/uapi/linux/kvm.h | 7 +++++++
> virt/kvm/arm/arm.c | 21 ++++++++++++++++++++
> virt/kvm/arm/mmio.c | 11 +++++++++--
> 9 files changed, 94 insertions(+), 2 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.txt b/Documentation/virt/kvm/api.txt
> index 2d067767b617..02501333f746 100644
> --- a/Documentation/virt/kvm/api.txt
> +++ b/Documentation/virt/kvm/api.txt
> @@ -4453,6 +4453,35 @@ Hyper-V SynIC state change. Notification is used to
> remap SynIC
> event/message pages and to enable/disable SynIC messages/events processing
> in userspace.
>
> + /* KVM_EXIT_ARM_NISV */
> + struct {
> + __u64 esr_iss;
> + __u64 fault_ipa;
> + } arm_nisv;
> +
> +Used on arm and arm64 systems. If a guest accesses memory not in a memslot,
> +KVM will typically return to userspace and ask it to do MMIO emulation on its
> +behalf. However, for certain classes of instructions, no instruction decode
> +(direction, length of memory access) is provided, and fetching and decoding
> +the instruction from the VM is overly complicated to live in the kernel.
> +
> +Historically, when this situation occurred, KVM would print a warning and
> kill
> +the VM. KVM assumed that if the guest accessed non-memslot memory, it was
> +trying to do I/O, which just couldn't be emulated, and the warning message
> was
> +phrased accordingly. However, what happened more often was that a guest bug
> +caused access outside the guest memory areas which should lead to a more
> +mearningful warning message and an external abort in the guest, if the access
meaningful?
> +did not fall within an I/O window.
> +
> +Userspace implementations can query for KVM_CAP_ARM_NISV_TO_USER, and enable
> +this capability at VM creation. Once this is done, these types of errors will
> +instead return to userspace with KVM_EXIT_ARM_NISV, with the valid bits from
> +the HSR (arm) and ESR_EL2 (arm64) in the esr_iss field, and the faulting IPA
> +in the fault_ipa field. Userspace can either fix up the access if it's
> +actually an I/O access by decoding the instruction from guest memory (if it's
> +very brave) and continue executing the guest, or it can decide to suspend,
> +dump, or restart the guest.
> +
> /* Fix the size of the union. */
> char padding[256];
> };
> diff --git a/arch/arm/include/asm/kvm_arm.h b/arch/arm/include/asm/kvm_arm.h
> index 0125aa059d5b..ce61b3b0058d 100644
> --- a/arch/arm/include/asm/kvm_arm.h
> +++ b/arch/arm/include/asm/kvm_arm.h
> @@ -162,6 +162,8 @@
> #define HSR_ISV (_AC(1, UL) << HSR_ISV_SHIFT)
> #define HSR_SRT_SHIFT (16)
> #define HSR_SRT_MASK (0xf << HSR_SRT_SHIFT)
> +#define HSR_CM (1 << 8)
> +#define HSR_WNR (1 << 6)
I think we already have that one (bizarrely placed after HSR_SSE).
> #define HSR_FSC (0x3f)
> #define HSR_FSC_TYPE (0x3c)
> #define HSR_SSE (1 << 21)
> diff --git a/arch/arm/include/asm/kvm_emulate.h
> b/arch/arm/include/asm/kvm_emulate.h
> index 40002416efec..e8ef349c04b4 100644
> --- a/arch/arm/include/asm/kvm_emulate.h
> +++ b/arch/arm/include/asm/kvm_emulate.h
> @@ -167,6 +167,11 @@ static inline bool kvm_vcpu_dabt_isvalid(struct kvm_vcpu
> *vcpu)
> return kvm_vcpu_get_hsr(vcpu) & HSR_ISV;
> }
>
> +static inline unsigned long kvm_vcpu_dabt_iss_nisv_sanitized(const struct
> kvm_vcpu *vcpu)
> +{
> + return kvm_vcpu_get_hsr(vcpu) & (HSR_CM | HSR_WNR | HSR_FSC);
> +}
> +
> static inline bool kvm_vcpu_dabt_iswrite(struct kvm_vcpu *vcpu)
> {
> return kvm_vcpu_get_hsr(vcpu) & HSR_WNR;
> diff --git a/arch/arm/include/asm/kvm_host.h b/arch/arm/include/asm/kvm_host.h
> index 8a37c8e89777..19a92c49039c 100644
> --- a/arch/arm/include/asm/kvm_host.h
> +++ b/arch/arm/include/asm/kvm_host.h
> @@ -76,6 +76,14 @@ struct kvm_arch {
>
> /* Mandated version of PSCI */
> u32 psci_version;
> +
> + /*
> + * If we encounter a data abort without valid instruction syndrome
> + * information, report this to user space. User space can (and
> + * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
> + * supported.
> + */
> + bool return_nisv_io_abort_to_user;
> };
>
> #define KVM_NR_MEM_OBJS 40
> diff --git a/arch/arm64/include/asm/kvm_emulate.h
> b/arch/arm64/include/asm/kvm_emulate.h
> index d69c1efc63e7..a3c967988e1d 100644
> --- a/arch/arm64/include/asm/kvm_emulate.h
> +++ b/arch/arm64/include/asm/kvm_emulate.h
> @@ -258,6 +258,11 @@ static inline bool kvm_vcpu_dabt_isvalid(const struct
> kvm_vcpu *vcpu)
> return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_ISV);
> }
>
> +static inline unsigned long kvm_vcpu_dabt_iss_nisv_sanitized(const struct
> kvm_vcpu *vcpu)
> +{
> + return kvm_vcpu_get_hsr(vcpu) & (ESR_ELx_CM | ESR_ELx_WNR |
> ESR_ELx_FSC);
> +}
> +
> static inline bool kvm_vcpu_dabt_issext(const struct kvm_vcpu *vcpu)
> {
> return !!(kvm_vcpu_get_hsr(vcpu) & ESR_ELx_SSE);
> diff --git a/arch/arm64/include/asm/kvm_host.h
> b/arch/arm64/include/asm/kvm_host.h
> index f656169db8c3..019bc560edc1 100644
> --- a/arch/arm64/include/asm/kvm_host.h
> +++ b/arch/arm64/include/asm/kvm_host.h
> @@ -83,6 +83,14 @@ struct kvm_arch {
>
> /* Mandated version of PSCI */
> u32 psci_version;
> +
> + /*
> + * If we encounter a data abort without valid instruction syndrome
> + * information, report this to user space. User space can (and
> + * should) opt in to this feature if KVM_CAP_ARM_NISV_TO_USER is
> + * supported.
> + */
> + bool return_nisv_io_abort_to_user;
> };
>
> #define KVM_NR_MEM_OBJS 40
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 5e3f12d5359e..dd79235b6435 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -235,6 +235,7 @@ struct kvm_hyperv_exit {
> #define KVM_EXIT_S390_STSI 25
> #define KVM_EXIT_IOAPIC_EOI 26
> #define KVM_EXIT_HYPERV 27
> +#define KVM_EXIT_ARM_NISV 28
>
> /* For KVM_EXIT_INTERNAL_ERROR */
> /* Emulate instruction failed. */
> @@ -392,6 +393,11 @@ struct kvm_run {
> } eoi;
> /* KVM_EXIT_HYPERV */
> struct kvm_hyperv_exit hyperv;
> + /* KVM_EXIT_ARM_NISV */
> + struct {
> + __u64 esr_iss;
> + __u64 fault_ipa;
> + } arm_nisv;
> /* Fix the size of the union. */
> char padding[256];
> };
> @@ -996,6 +1002,7 @@ struct kvm_ppc_resize_hpt {
> #define KVM_CAP_ARM_PTRAUTH_ADDRESS 171
> #define KVM_CAP_ARM_PTRAUTH_GENERIC 172
> #define KVM_CAP_PMU_EVENT_FILTER 173
> +#define KVM_CAP_ARM_NISV_TO_USER 174
>
> #ifdef KVM_CAP_IRQ_ROUTING
>
> diff --git a/virt/kvm/arm/arm.c b/virt/kvm/arm/arm.c
> index 35a069815baf..7153504bb106 100644
> --- a/virt/kvm/arm/arm.c
> +++ b/virt/kvm/arm/arm.c
> @@ -98,6 +98,26 @@ int kvm_arch_check_processor_compat(void)
> return 0;
> }
>
> +int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> + struct kvm_enable_cap *cap)
> +{
> + int r;
> +
> + if (cap->flags)
> + return -EINVAL;
> +
> + switch (cap->cap) {
> + case KVM_CAP_ARM_NISV_TO_USER:
> + r = 0;
> + kvm->arch.return_nisv_io_abort_to_user = true;
> + break;
> + default:
> + r = -EINVAL;
> + break;
> + }
> +
> + return r;
> +}
>
> /**
> * kvm_arch_init_vm - initializes a VM data structure
> @@ -196,6 +216,7 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long
> ext)
> case KVM_CAP_MP_STATE:
> case KVM_CAP_IMMEDIATE_EXIT:
> case KVM_CAP_VCPU_EVENTS:
> + case KVM_CAP_ARM_NISV_TO_USER:
> r = 1;
> break;
> case KVM_CAP_ARM_SET_DEVICE_ADDR:
> diff --git a/virt/kvm/arm/mmio.c b/virt/kvm/arm/mmio.c
> index 6af5c91337f2..7b92e2744fa7 100644
> --- a/virt/kvm/arm/mmio.c
> +++ b/virt/kvm/arm/mmio.c
> @@ -167,8 +167,15 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run
> *run,
> if (ret)
> return ret;
> } else {
> - kvm_err("load/store instruction decoding not implemented\n");
> - return -ENOSYS;
> + if (vcpu->kvm->arch.return_nisv_io_abort_to_user) {
> + run->exit_reason = KVM_EXIT_ARM_NISV;
> + run->arm_nisv.esr_iss =
> kvm_vcpu_dabt_iss_nisv_sanitized(vcpu);
> + run->arm_nisv.fault_ipa = fault_ipa;
> + return 0;
> + } else {
> + kvm_info("Encountered data abort outside memslots with
> no valid syndrome info\n");
Nit: you could lose the else clause, making this line a bit shorter.
More importantly, I'd like this to become a rate-limited output, just to
keep things sane. How about kvm_pr_unimpl()?
> + return -ENOSYS;
> + }
> }
>
> rt = vcpu->arch.mmio_decode.rt;
>
Other than this collection of nits, this looks pretty good, and I'd
really like to take this patch in 5.5.
Thanks,
M.
--
Jazz is not dead, it just smells funny...
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