KVM: fix bad user ABI for KVM_EXIT_SYSTEM_EVENT

When KVM_EXIT_SYSTEM_EVENT was introduced, it included a flags
member that at the time was unused.  Unfortunately this extensibility
mechanism has several issues:

- x86 is not writing the member, so it would not be possible to use it
  on x86 except for new events

- the member is not aligned to 64 bits, so the definition of the
  uAPI struct is incorrect for 32- on 64-bit userspace.  This is a
  problem for RISC-V, which supports CONFIG_KVM_COMPAT, but fortunately
  usage of flags was only introduced in 5.18.

Since padding has to be introduced, place a new field in there
that tells if the flags field is valid.  To allow further extensibility,
in fact, change flags to an array of 16 values, and store how many
of the values are valid.  The availability of the new ndata field
is tied to a system capability; all architectures are changed to
fill in the field.

To avoid breaking compilation of userspace that was using the flags
field, provide a userspace-only union to overlap flags with data[0].
The new field is placed at the same offset for both 32- and 64-bit
userspace.

Cc: Will Deacon <will@kernel.org>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Peter Gonda <pgonda@google.com>
Cc: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reported-by: kernel test robot <lkp@intel.com>
Message-Id: <20220422103013.34832-1-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 17 insertions, 7 deletions

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 85c7abc51af5..4a900cdbc62e 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -5986,16 +5986,16 @@ should put the acknowledged interrupt vector into the 'epr' field.
   #define KVM_SYSTEM_EVENT_RESET          2
   #define KVM_SYSTEM_EVENT_CRASH          3
 			__u32 type;
-			__u64 flags;
+                        __u32 ndata;
+                        __u64 data[16];
 		} system_event;
 
 If exit_reason is KVM_EXIT_SYSTEM_EVENT then the vcpu has triggered
 a system-level event using some architecture specific mechanism (hypercall
 or some special instruction). In case of ARM64, this is triggered using
-HVC instruction based PSCI call from the vcpu. The 'type' field describes
-the system-level event type. The 'flags' field describes architecture
-specific flags for the system-level event.
+HVC instruction based PSCI call from the vcpu.
 
+The 'type' field describes the system-level event type.
 Valid values for 'type' are:
 
  - KVM_SYSTEM_EVENT_SHUTDOWN -- the guest has requested a shutdown of the
@@ -6010,10 +6010,20 @@ Valid values for 'type' are:
    to ignore the request, or to gather VM memory core dump and/or
    reset/shutdown of the VM.
 
-Valid flags are:
+If KVM_CAP_SYSTEM_EVENT_DATA is present, the 'data' field can contain
+architecture specific information for the system-level event.  Only
+the first `ndata` items (possibly zero) of the data array are valid.
 
- - KVM_SYSTEM_EVENT_RESET_FLAG_PSCI_RESET2 (arm64 only) -- the guest issued
-   a SYSTEM_RESET2 call according to v1.1 of the PSCI specification.
+ - for arm64, data[0] is set to KVM_SYSTEM_EVENT_RESET_FLAG_PSCI_RESET2 if
+   the guest issued a SYSTEM_RESET2 call according to v1.1 of the PSCI
+   specification.
+
+ - for RISC-V, data[0] is set to the value of the second argument of the
+   ``sbi_system_reset`` call.
+
+Previous versions of Linux defined a `flags` member in this struct.  The
+field is now aliased to `data[0]`.  Userspace can assume that it is only
+written if ndata is greater than 0.
 
 ::