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Date:   Thu, 29 Jun 2023 14:16:00 +0300
From:   kirill.shutemov@linux.intel.com
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Kai Huang <kai.huang@intel.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, linux-mm@kvack.org, x86@kernel.org,
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Subject: Re: [PATCH v12 20/22] x86/virt/tdx: Allow SEAMCALL to handle #UD and
 #GP
Message-ID: <20230629111600.hq4carptan6pfu37@box.shutemov.name>
References: <cover.1687784645.git.kai.huang@intel.com>
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On Wed, Jun 28, 2023 at 10:38:23PM +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.
> 
> 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.
> 
> 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.

I've just brought it up again internally. Let's see what will happen.
 

-- 
  Kiryl Shutsemau / Kirill A. Shutemov