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Date:   Wed, 28 Jun 2023 22:38:23 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Kai Huang <kai.huang@intel.com>
Cc:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        linux-mm@kvack.org, x86@kernel.org, dave.hansen@intel.com,
        kirill.shutemov@linux.intel.com, tony.luck@intel.com,
        tglx@linutronix.de, bp@alien8.de, mingo@redhat.com, hpa@zytor.com,
        seanjc@google.com, pbonzini@redhat.com, david@redhat.com,
        dan.j.williams@intel.com, rafael.j.wysocki@intel.com,
        ashok.raj@intel.com, reinette.chatre@intel.com,
        len.brown@intel.com, ak@linux.intel.com, isaku.yamahata@intel.com,
        ying.huang@intel.com, chao.gao@intel.com,
        sathyanarayanan.kuppuswamy@linux.intel.com, nik.borisov@suse.com,
        bagasdotme@gmail.com, sagis@google.com, imammedo@redhat.com
Subject: Re: [PATCH v12 20/22] x86/virt/tdx: Allow SEAMCALL to handle #UD and
 #GP
Message-ID: <20230628203823.GR38236@hirez.programming.kicks-ass.net>
References: <cover.1687784645.git.kai.huang@intel.com>
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 <20230628152900.GI2438817@hirez.programming.kicks-ass.net>
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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.