Re: TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and syscall trainwreck in hardware)

[Dave Hansen]

On 8/24/20 9:39 PM, Sean Christopherson wrote:
> +Andy
> 
> On Mon, Aug 24, 2020 at 02:52:01PM +0100, Andrew Cooper wrote:
>> And to help with coordination, here is something prepared (slightly)
>> earlier.
>>
>> https://docs.google.com/document/d/1hWejnyDkjRRAW-JEsRjA5c9CKLOPc6VKJQsuvODlQEI/edit?usp=sharing
>>
>> This identifies the problems from software's perspective, along with
>> proposing behaviour which ought to resolve the issues.
>>
>> It is still a work-in-progress.  The #VE section still needs updating in
>> light of the publication of the recent TDX spec.
> 
> For #VE on memory accesses in the SYSCALL gap (or NMI entry), is this
> something we (Linux) as the guest kernel actually want to handle gracefully
> (where gracefully means not panicking)?  For TDX, a #VE in the SYSCALL gap
> would require one of two things:
> 
>   a) The guest kernel to not accept/validate the GPA->HPA mapping for the
>      relevant pages, e.g. code or scratch data.
> 
>   b) The host VMM to remap the GPA (making the GPA->HPA pending again).
> 
> (a) is only possible if there's a fatal buggy guest kernel (or perhaps vBIOS).
> (b) requires either a buggy or malicious host VMM.
> 
> I ask because, if the answer is "no, panic at will", then we shouldn't need
> to burn an IST for TDX #VE.  Exceptions won't morph to #VE and hitting an
> instruction based #VE in the SYSCALL gap would be a CPU bug or a kernel bug.
> Ditto for a #VE in NMI entry before it gets to a thread stack.
> 
> Am I missing anything?

No, that was my expectation as well.  My only concern is that someone
might unintentionally put a #VE'ing instruction in one of the tricky
entry paths, like if we decided we needed CPUID for serialization or
used a WRMSR that #VE's.

It's just something we need to look out for when mucking in the entry
paths.  But, it's not that hard given how few things actually #VE.