Re: [PATCH] x86/tdx: Generate SIGBUS on userspace MMIO

[Dave Hansen <dave.hansen@intel.com>]

On 6/10/24 06:55, Dave Hansen wrote:
>> Enlightened userspace may choose to handle MMIO on their own if the
>> kernel does not emulate it.
>>
>> Handle the EPT_VIOLATION exit reason for userspace and deliver SIGBUS
>> instead of SIGSEGV. SIGBUS is more appropriate for the MMIO situation.
> Is any userspace _actually_ doing this?  Sure, SIGBUS is more
> appropriate but in practice unprepared userspace crashes either way.

I also can't help but wonder if there's a better way to do this.

Just thinking out loud.... Ideally, we'd reject creating a potentially
troublesome VMA at mmap() time.  That's way better than, for instance,
panic()'ing at some random place in the middle of program execution.

But I guess that's likely not possible because someone could be doing a
VM_MIXEDMAP VMA that only has normal private pages and never _actually_
needs or has a shared page mapped.

I'd still love to know what actual kernel drivers and actual userspace
would be involved in this whole dance.  It's still way too theoretical
for me.