Re: [RFC v1 05/26] x86/traps: Add #VE support for TDX guest

[Andy Lutomirski]

On Fri, Feb 12, 2021 at 1:37 PM Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>
> On Fri, Feb 12, 2021, Dave Hansen wrote:
> > On 2/12/21 12:54 PM, Sean Christopherson wrote:
> > > Ah, I see what you're thinking.
> > >
> > > Treating an EPT #VE as fatal was also considered as an option.  IIUC it was
> > > thought that finding every nook and cranny that could access a page, without
> > > forcing the kernel to pre-accept huge swaths of memory, would be very difficult.
> > > It'd be wonderful if that's not the case.
> >
> > We have to manually set up the page table entries for every physical
> > page of memory (except for the hard-coded early stuff below 8MB or
> > whatever).  We *KNOW*, 100% before physical memory is accessed.
> >
> > There aren't nooks and crannies where memory is accessed.  There are a
> > few, very well-defined choke points which must be crossed before memory
> > is accessed.  Page table creation, bootmem and the core page allocator
> > come to mind.
>
> Heh, for me, that's two places too many beyond my knowledge domain to feel
> comfortable putting a stake in the ground saying #VE isn't necessary.
>
> Joking aside, I agree that treating EPT #VEs as fatal would be ideal, but from a
> TDX architecture perspective, when considering all possible kernels, drivers,
> configurations, etc..., it's risky to say that there will _never_ be a scenario
> that "requires" #VE.
>
> What about adding a property to the TD, e.g. via a flag set during TD creation,
> that controls whether unaccepted accesses cause #VE or are, for all intents and
> purposes, fatal?  That would allow Linux to pursue treating EPT #VEs for private
> GPAs as fatal, but would give us a safety and not prevent others from utilizing
> #VEs.

That seems reasonable.