Re: [RFC v2-fix-v2 2/2] x86/tdx: Ignore WBINVD instruction for TDX guest

[Andi Kleen]

On 5/24/2021 8:40 PM, Dan Williams wrote:
> On Mon, May 24, 2021 at 8:27 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> > On 5/24/2021 7:49 PM, Dan Williams wrote:
> > > On Mon, May 24, 2021 at 7:13 PM Andi Kleen <ak@xxxxxxxxxxxxxxx> wrote:
> > > [..]
> > > > > ...to explicitly error out a wbinvd use case before data is altered
> > > > > and wbinvd is needed.
> > > > I don't see any point of all of this. We really just want to be the same
> > > > as KVM. Not get into the business of patching a bazillion sub systems
> > > > that cannot be used in TDX anyways.
> > > Please let's not start this patch off with dubious claims of safety
> > > afforded by IgnorePAT. Instead make the true argument that wbinvd is
> > > known to be problematic in guests
> > That's just another reason to not support WBINVD, but I don't think it's
> > the main reason. The main reason is that it is simply not needed, unless
> > you do DMA in some form.
> >
> > (and yes I consider direct mapping of persistent memory with a complex
> > setup procedure a form of DMA -- my guess is that the reason that it
> > works in KVM is that it somehow activates the DMA code paths in KVM)
> No, it doesn't. Simply no one has tried to pass through the security
> interface of bare metal nvdimm to a guest, or enabled the security
> commands in a virtualized nvdimm.

Maybe a better term would be "external side effects". If you have 
something in IO domain which can notice a difference.

> If a guest supports a memory map it supports PMEM I struggle to see DMA anywhere in that equation.

Okay if that's happen to a TDX guest we have to start emulate WBINVD. 
But right now we don't need it.

I guess we can add a comment that says

"if someone wants to implement NVDIMM secure delete they would also need 
to implement this new hypercall"

> > IMNSHO that's the true reason.
> I do see why it would be attractive if IgnorePAT was a solid signal to
> ditch wbinvd support. However, it simply isn't, and to date nothing
> has cared trip over that gap.


I think we're getting into angels on a pinhead here.

The key point is that current TDX does not need WBINVD. I believe we 
agree on that.


-Andi