Re: [PATCH v5 11/12] x86/tdx: Don't write CSTAR MSR on Intel

Dave Hansen
2021-08-04T21:48:43Z

On 8/4/21 2:03 PM, Kuppuswamy, Sathyanarayanan wrote:
>> Is #GP the actual TDX-Module behavior?  If so, isn't that a
>> contradiction with
>
> No, #GP is triggered by guest.
...
>> Regardless of #GP versus #VE, "Table 16.2 MSR Virtualization" needs
>> to state the actual behavior.
>
> Even in this case, it will trigger #VE. But since CSTAR MSR is not
> supported, write to it will fail and leads to #VE fault.

Sathya, I think there might be a mixup of terminology here that's
confusing. I'm confused by this exchange.

In general, we refer to hardware exceptions by their architecture names:
#GP for general protection fault, #PF for page fault, #VE for
Virtualization Exception.

Those hardware exceptions are wired up to software handlers:
#GP lands in asm_exc_general_protection
#PF ends up in exc_page_fault
#VE ends up in exc_virtualization_exception
... and more of course

But, to add to the confusion, the #VE handler
(exc_virtualization_exception()) itself calls (or did once upon a time
call) do_general_protection() when it can't handle something.
do_general_protection() is (was?) *ALSO* called by the #GP handler.

So, is that what you meant? By "#GP is triggered by guest", you mean
that a write to the CSTAR MSR and the resulting #VE will end up being
handled in a way that is similar to how a #GP hardware exception would
have been handled?

If that's what you meant, I'm not _sure_ that's totally accurate. Could
you elaborate on this a bit? It also would be really handy if you were
able to adopt the terminology I talked about above. It will really make
things less confusing.