Re: [PATCH v12 20/22] x86/virt/tdx: Allow SEAMCALL to handle #UD and #GP

[Huang, Kai]

On Wed, 2023-06-28 at 22:38 +0200, Peter Zijlstra wrote:
> On Wed, Jun 28, 2023 at 05:29:01PM +0200, Peter Zijlstra wrote:
> > On Tue, Jun 27, 2023 at 02:12:50AM +1200, Kai Huang wrote:
> > > diff --git a/arch/x86/virt/vmx/tdx/tdxcall.S b/arch/x86/virt/vmx/tdx/tdxcall.S
> > > index 49a54356ae99..757b0c34be10 100644
> > > --- a/arch/x86/virt/vmx/tdx/tdxcall.S
> > > +++ b/arch/x86/virt/vmx/tdx/tdxcall.S
> > > @@ -1,6 +1,7 @@
> > >  /* SPDX-License-Identifier: GPL-2.0 */
> > >  #include <asm/asm-offsets.h>
> > >  #include <asm/tdx.h>
> > > +#include <asm/asm.h>
> > >  
> > >  /*
> > >   * TDCALL and SEAMCALL are supported in Binutils >= 2.36.
> > > @@ -45,6 +46,7 @@
> > >  	/* Leave input param 2 in RDX */
> > >  
> > >  	.if \host
> > > +1:
> > >  	seamcall
> > 
> > So what registers are actually clobbered by SEAMCALL ? There's a
> > distinct lack of it in SDM Vol.2 instruction list :-(
> 
> With the exception of the abomination that is TDH.VP.ENTER all SEAMCALLs
> seem to be limited to the set presented here (c,d,8,9,10,11) and all
> other registers should be available.

RAX is also used as SEAMCALL return code.

Looking at the later versions of TDX spec (with TD live migration, etc), it
seems they are already using R12-R13 as SEAMCALL output:

https://cdrdv2.intel.com/v1/dl/getContent/733579

E.g., 6.3.15. NEW: TDH.IMPORT.MEM Leaf

It uses R12 and R13 as input.

> 
> Can we please make that a hard requirement, SEAMCALL must not use
> registers outside this? We can hardly program to random future
> extentions; we need hard ABI guarantees here.


I believe all other GPRs are just saved/restored in SEAMCALL/SEAMRET, so in
practice all other GPRs not used as input/output should not be clobbered.  But I
will confirm with TDX module guys.  And even it's true in practice it's better
to document it.  

But I think we also want to ask them to stop adding more registers as
input/output.

I'll talk to TDX module team on this.

> 
> That also means we should be able to use si,di for the cmovc below.
> 
> Kirill, back when we did __tdx_hypercall() we got bp removed as a valid
> register, the 1.0 spec still lists that, and it is also listed in
> TDH.VP.ENTER, I'm assuming it will be removed there too?
> 
> bp must not be used -- it violates the pre-existing calling convention.
> 
>