Re: [RFC v2-fix 1/1] x86/tdx: Handle in-kernel MMIO

[Andi Kleen]

On 5/18/2021 9:10 AM, Andi Kleen wrote:
> > > > > For now we only handle a subset of instructions that the kernel
> > > > > uses for MMIO operations. User-space access triggers SIGBUS.
> > > > How do you know which instructions the kernel uses?
> > > They're all in MMIO macros.
> > I've heard exactly the opposite from the TDX team in the past. What I
> > remember was a claim that one can not just leverage the MMIO macros as a
> > single point to avoid MMIO.  I remember being told that not all code in
> > the kernel that does MMIO uses these macros.  APIC MMIO's were called
> > out as a place that does not use the MMIO macros.
>
> Yes x86 APIC has its own macros, but we don't use the MMIO based APIC, 
> only X2APIC in TDX.

I must correct myself here. We actually use #VE to handle MSRs, or at 
least those that are not context switched by the TDX module. So there 
can be #VE nested in NMI in normal operation, since MSR accesses in NMI 
can happen.

I don't think it needs any changes to the code -- this should all work 
-- but we need to update the commit log to document this case.


-Andi