Re: [PATCH 0/4] KVM: fix KVM_EXIT_SYSTEM_EVENT mess

Oliver Upton
2022-04-22T07:58:36Z

Hi Paolo,

On Thu, Apr 21, 2022 at 02:04:39PM -0400, Paolo Bonzini wrote:
> The KVM_SYSTEM_EVENT_NDATA_VALID mechanism that was introduced
> contextually with KVM_SYSTEM_EVENT_SEV_TERM is not a good match
> for ARM and RISC-V, which want to communicate information even
> for existing KVM_SYSTEM_EVENT_* constants. Userspace is not ready
> to filter out bit 31 of type, and fails to process the
> KVM_EXIT_SYSTEM_EVENT exit.
>
> Therefore, tie the availability of ndata to a system capability;
> if the capability is present, ndata is always valid, so patch 1
> makes x86 always initialize it. Then patches 2 and 3 fix
> ARM and RISC-V compilation and patch 4 enables the capability.
>
> Only compiled on x86, waiting for acks.
>
> Paolo
>
> Paolo Bonzini (4):
> KVM: x86: always initialize system_event.ndata
> KVM: ARM: replace system_event.flags with ndata and data[0]
> KVM: RISC-V: replace system_event.flags with ndata and data[0]
> KVM: tell userspace that system_event.ndata is valid

Is there any way we could clean this up in 5.18 and leave the whole
ndata/data pattern for 5.19?

IOW, for 5.18 go back and fix the padding:

struct {
__u32 type;
__u32 pad;
__u64 flags;
} system_event;

Then for 5.19 circle back on the data business, except use a flag bit
for it:

struct {
__u32 type;
__u32 pad;
#define KVM_SYSTEM_EVENT_NDATA_VALID (1u << 63)
__u64 flags;
__u64 ndata;
__u64 data[16];
} system_event;

Where we apply that bit to system_event::flags this time instead of
::type. Could also go the CAP route.

Wouldn't this be enough to preserve ABI with whatever userspace has been
spun up for system_event::flags already and also keep the SEV stuff
happy in 5.19?

It is a bit weird to churn existing UAPI usage in the very next kernel
cycle, but could be convinced otherwise :)

--
Thanks,
Oliver