Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more TDCALL/SEAMCALL leafs

[Peter Zijlstra]

On Thu, Jul 13, 2023 at 09:34:24AM +0000, Huang, Kai wrote:
> On Thu, 2023-07-13 at 10:46 +0200, Peter Zijlstra wrote:
> > On Thu, Jul 13, 2023 at 07:48:20AM +0000, Huang, Kai wrote:
> > 
> > > I found below comment in KVM code:
> > > 
> > > > +	 * See arch/x86/kvm/vmx/vmenter.S:
> > > > +	 *
> > > > +	 * In theory, a L1 cache miss when restoring register from stack
> > > > +	 * could lead to speculative execution with guest's values.
> > > 
> > > And KVM explicitly does XOR for the registers that gets "pop"ed almost
> > > instantly, so I followed.
> > > 
> > > But to be honest I don't quite understand this.  :-)
> > 
> > Urgh, I suppose that actually makes sense. Since pop is a load it might
> > continue speculation with the previous value. Whereas the xor-clear
> > idiom is impossible to speculate through.
> > 
> > Oh well...
> 
> Then should I keep those registers that are "pop"ed immediately afterwards?

Yeah, I suppose so.