Messages in this thread | ![/](/images/icornerl.gif) | | Date | Mon, 24 Aug 2020 21:39:59 -0700 | From | Sean Christopherson <> | Subject | TDX #VE in SYSCALL gap (was: [RFD] x86: Curing the exception and syscall trainwreck in hardware) |
| |
+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?
| ![\](/images/icornerr.gif) |