Re: [PATCH v2 03/21] x86/virt/tdx: Implement the SEAMCALL base function

From: Kai Huang <kai.huang@intel.com>
To: "Tian, Kevin" <kevin.tian@intel.com>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"kvm@vger.kernel.org" <kvm@vger.kernel.org>
Cc: "Hansen, Dave" <dave.hansen@intel.com>, "Christopherson,,
	Sean" <seanjc@google.com>,
	"pbonzini@redhat.com" <pbonzini@redhat.com>,
	"kirill.shutemov@linux.intel.com"
	<kirill.shutemov@linux.intel.com>,
	"sathyanarayanan.kuppuswamy@linux.intel.com" 
	<sathyanarayanan.kuppuswamy@linux.intel.com>,
	"peterz@infradead.org" <peterz@infradead.org>,
	"Luck, Tony" <tony.luck@intel.com>,
	"ak@linux.intel.com" <ak@linux.intel.com>,
	"Williams, Dan J" <dan.j.williams@intel.com>,
	"Yamahata, Isaku" <isaku.yamahata@intel.com>
Subject: Re: [PATCH v2 03/21] x86/virt/tdx: Implement the SEAMCALL base function
Date: Mon, 28 Mar 2022 14:41:32 +1300	[thread overview]
Message-ID: <926af8966a2233574ee0e679d9fc3c8209477156.camel@intel.com> (raw)
In-Reply-To: <BN9PR11MB5276B5986582F9AD11D993618C189@BN9PR11MB5276.namprd11.prod.outlook.com>

On Wed, 2022-03-23 at 16:35 +1300, Tian, Kevin wrote:
> > From: Kai Huang <kai.huang@intel.com>
> > Sent: Sunday, March 13, 2022 6:50 PM
> > 
> > Secure Arbitration Mode (SEAM) is an extension of VMX architecture.  It
> > defines a new VMX root operation (SEAM VMX root) and a new VMX non-
> > root
> > operation (SEAM VMX non-root) which isolate from legacy VMX root and
> > VMX
> > non-root mode.
> 
> s/isolate/are isolated/

OK thanks.

> 
> > 
> > A CPU-attested software module (called the 'TDX module') runs in SEAM
> > VMX root to manage the crypto protected VMs running in SEAM VMX non-
> > root.
> > SEAM VMX root is also used to host another CPU-attested software module
> > (called the 'P-SEAMLDR') to load and update the TDX module.
> > 
> > Host kernel transits to either the P-SEAMLDR or the TDX module via the
> > new SEAMCALL instruction.  SEAMCALLs are host-side interface functions
> > defined by the P-SEAMLDR and the TDX module around the new SEAMCALL
> > instruction.  They are similar to a hypercall, except they are made by
> 
> "SEAMCALLs are ... functions ... around the new SEAMCALL instruction"
> 
> This is confusing. Probably just:

May I ask why is it confusing?

> 
> "SEAMCALL functions are defined and handled by the P-SEAMLDR and
> the TDX module"
> 
> > host kernel to the SEAM software.
> > 
> > SEAMCALLs use an ABI different from the x86-64 system-v ABI.  Instead,
> > they share the same ABI with the TDCALL.  %rax is used to carry both the
> > SEAMCALL leaf function number (input) and the completion status code
> > (output).  Additional GPRs (%rcx, %rdx, %r8->%r11) may be further used
> > as both input and output operands in individual leaf SEAMCALLs.
> > 
> > Implement a C function __seamcall() to do SEAMCALL using the assembly
> > macro used by __tdx_module_call() (the implementation of TDCALL).  The
> > only exception not covered here is TDENTER leaf function which takes
> > all GPRs and XMM0-XMM15 as both input and output.  The caller of TDENTER
> > should implement its own logic to call TDENTER directly instead of using
> > this function.
> > 
> > SEAMCALL instruction is essentially a VMExit from VMX root to SEAM VMX
> > root, and it can fail with VMfailInvalid, for instance, when the SEAM
> > software module is not loaded.  The C function __seamcall() returns
> > TDX_SEAMCALL_VMFAILINVALID, which doesn't conflict with any actual error
> > code of SEAMCALLs, to uniquely represent this case.
> 
> SEAMCALL is TDX specific, is it? If yes, there is no need to have both
> TDX and SEAMCALL in one macro, i.e. above can be SEAMCALL_VMFAILINVALID.

This is defined in TDX guest series.  I just use it.

https://lore.kernel.org/lkml/20220324152415.grt6xvhblmd4uccu@black.fi.intel.com/T/#md0b1aa563bd003ab625de159612a0d07e3ded7cb

-- 
Thanks,
-Kai