Re: [PATCH v5 22/34] x86/fred: FRED initialization code

[Dave Hansen]

On 3/21/23 19:22, Li, Xin3 wrote:
>> If there is no other concrete reason other than overflowing for assigning NMI and
>> #DB with a stack level > 0, #VE should also be assigned with a stack level > 0, and
>> #BP too. #VE can happen anytime and anywhere, so it is subject to overflowing too.
> With IDT, both #VE and #BP do not use IST, but NMI, #DB, #MC and #DF do.
> 
> Let's keep this "secret" logic for now, i.e., not change the stack levels
> for #VE and #BP at this point. We can do "optimization", i.e., change them
> later 😄.

#VE also can't happen anywhere.  There is some documentation about it in
here:

	https://docs.kernel.org/x86/tdx.html#linux-ve-handler

But, basically, the only halfway sane thing a guest might do to hit a
#VE is touch some "MMIO".  The host can *not* cause them in arbitrary
places because of the SEPT_VE_DISABLE attribute.

#VE's also can't nest until after the guest retrieves the "VE info".
That means that the #VE handler at _least_ reaches C code before it's
subject to another #VE and that second one would still need to be
induced by something the guest does explicitly.