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

["Huang, Kai" <kai.huang@intel.com>]

On Fri, 2023-06-30 at 12:21 +0200, Peter Zijlstra wrote:
> On Fri, Jun 30, 2023 at 12:07:00PM +0200, Peter Zijlstra wrote:
> > On Thu, Jun 29, 2023 at 10:33:38AM +0000, Huang, Kai wrote:
> 
> > > 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
> > 
> > Urgh.. I think I read an older versio because I got bleeding eyes from
> > all this colour coded crap.
> > 
> > All this red is unreadable :-( Have they been told about the glories of
> > TeX and diff ?
> > 
> > > E.g., 6.3.15. NEW: TDH.IMPORT.MEM Leaf
> > > 
> > > It uses R12 and R13 as input.
> > 
> > 12 and 14. They skipped 13 for some mysterious raisin.
> 
> Things like TDH.SERVTD.BIND do use R13.
> 
> > But also, 10,11 are frequently used as input with this new stuff, which
> > already suggests the setup from your patches is not tenable.
> 
> 
> TDG.SERVTD.RD *why* can't they pass that TD_UUID as a pointer? Using *4*
> registers like that is quite insane.

I can ask TDX module team whether they can change to use a buffer instead of 4
registers.

> 
> TDG.VP.ENTER :-(((( that has b,15,si,di as additional output.
> 
> That means there's not a single register left unused. Can we still get
> this changed, please?!?
> 

I assume it's TDH.VP.ENTER.

TDH.VP.ENTER is a special SEAMCALL because it goes into TDX guest to run.  KVM
should be the only place that handles it.  And KVM won't use the __seamcall()
here to handle it but will have it's own assembly.

For normal VMX guests, since most GPRs are not saved/restored by hardware, KVM
already has sufficient code to save/restore relevant GPRs during VMENTER/VMEXIT.
Handling TDX can just follow that pattern.

So looks fine to me.