Re: [PATCH 2/2] KVM: arm/arm64: Allow user injection of external data aborts

[Peter Maydell <peter.maydell@linaro.org>]

On Mon, 9 Sep 2019 at 13:13, Christoffer Dall <christoffer.dall@arm.com> wrote:
>
> In some scenarios, such as buggy guest or incorrect configuration of the
> VMM and firmware description data, userspace will detect a memory access
> to a portion of the IPA, which is not mapped to any MMIO region.
>
> For this purpose, the appropriate action is to inject an external abort
> to the guest.  The kernel already has functionality to inject an
> external abort, but we need to wire up a signal from user space that
> lets user space tell the kernel to do this.
>
> It turns out, we already have the set event functionality which we can
> perfectly reuse for this.
>
> Signed-off-by: Christoffer Dall <christoffer.dall@arm.com>
> ---
>  Documentation/virt/kvm/api.txt    | 15 ++++++++++++++-
>  arch/arm/include/uapi/asm/kvm.h   |  3 ++-
>  arch/arm/kvm/guest.c              |  3 +++
>  arch/arm64/include/uapi/asm/kvm.h |  3 ++-
>  arch/arm64/kvm/guest.c            |  3 +++
>  arch/arm64/kvm/inject_fault.c     |  4 ++--
>  include/uapi/linux/kvm.h          |  1 +
>  virt/kvm/arm/arm.c                |  1 +
>  8 files changed, 28 insertions(+), 5 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.txt b/Documentation/virt/kvm/api.txt
> index 02501333f746..edd6cdc470ca 100644
> --- a/Documentation/virt/kvm/api.txt
> +++ b/Documentation/virt/kvm/api.txt
> @@ -955,6 +955,8 @@ The following bits are defined in the flags field:
>
>  ARM/ARM64:
>
> +User space may need to inject several types of events to the guest.
> +
>  If the guest accesses a device that is being emulated by the host kernel in
>  such a way that a real device would generate a physical SError, KVM may make
>  a virtual SError pending for that VCPU. This system error interrupt remains
> @@ -989,12 +991,23 @@ Specifying exception.has_esr on a system that does not support it will return
>  -EINVAL. Setting anything other than the lower 24bits of exception.serror_esr
>  will return -EINVAL.
>
> +If the guest performed an access to I/O memory which could not be handled by
> +user space, for example because of missing instruction syndrome decode
> +information or because there is no device mapped at the accessed IPA, then
> +user space can ask the kernel to inject an external abort using the address
> +from the exiting fault on the VCPU. It is a programming error to set
> +ext_dabt_pending at the same time as any of the serror fields, or to set
> +ext_dabt_pending on an exit which was not either KVM_EXIT_MMIO or
> +KVM_EXIT_ARM_NISV. This feature is only available if the system supports
> +KVM_CAP_ARM_INJECT_EXT_DABT;
> +
>  struct kvm_vcpu_events {
>         struct {
>                 __u8 serror_pending;
>                 __u8 serror_has_esr;
> +               __u8 ext_dabt_pending;
>                 /* Align it to 8 bytes */
> -               __u8 pad[6];
> +               __u8 pad[5];
>                 __u64 serror_esr;
>         } exception;
>         __u32 reserved[12];

This API seems to be missing support for userspace to specify
whether the ESR_ELx for the guest should have the EA bit set
(and more generally other syndrome/fault status bits). I think
if we have an API for "KVM_EXIT_MMIO but the access failed"
then it should either (a) be architecture agnostic, since
pretty much any architecture might have a concept of "access
gave some bus-error-type failure" and it would be nice if userspace
didn't have to special case them all in arch-specific code,
or (b) have the same flexibility for specifying exactly what
kind of fault as the architecture does. This sort of seems to
fall between two stools. (My ideal for KVM_EXIT_MMIO faults
would be a generic API which included space for optional
arch-specific info, which for Arm would pretty much just be
the EA bit.)

As and when we support nested virtualization, any suggestions
on how this API would extend to support userspace saying
"deliver fault to guest EL1" vs "deliver fault to guest EL2" ?

thanks
-- PMM
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