Re: [PATCH 1/2] KVM: arm/arm64: Allow reporting non-ISV data aborts to userspace

[James Morse <james.morse@arm.com>]

Hi Christoffer,

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.



> 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
> +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.

Should we document which parts of instruction-emulation the VMM has to do?

For KVM_EXIT_MMIO, kvm looks after updating registers and advancing the PC and SS state
machine.

I can't see a kvm_skip_instr() in here, so the VMM has to do all of that stuff, including
any register post-increment, which is the reason we need the instruction in the first place.


Thanks,

James
_______________________________________________
kvmarm mailing list
kvmarm@lists.cs.columbia.edu
https://lists.cs.columbia.edu/mailman/listinfo/kvmarm