linux-kernel - Re: [PATCH 09/10] x86/virt/tdx: Wire up basic SEAMCALL functions

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Date:   Thu, 13 Jul 2023 08:18:09 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "peterz@...radead.org" <peterz@...radead.org>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>, "x86@...nel.org" <x86@...nel.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "isaku.yamahata@...il.com" <isaku.yamahata@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH 09/10] x86/virt/tdx: Wire up basic SEAMCALL functions

On Thu, 2023-07-13 at 09:42 +0200, Peter Zijlstra wrote:
> On Thu, Jul 13, 2023 at 03:46:52AM +0000, Huang, Kai wrote:
> > On Wed, 2023-07-12 at 15:15 -0700, Isaku Yamahata wrote:
> > > > The SEAMCALL ABI is very similar to the TDCALL ABI and leverages much
> > > > TDCALL infrastructure.  Wire up basic functions to make SEAMCALLs for
> > > > the basic TDX support: __seamcall(), __seamcall_ret() and
> > > > __seamcall_saved_ret() which is for TDH.VP.ENTER leaf function.
> > > 
> > > Hi.  __seamcall_saved_ret() uses struct tdx_module_arg as input and output.  For
> > > KVM TDH.VP.ENTER case, those arguments are already in unsigned long
> > > kvm_vcpu_arch::regs[].  It's silly to move those values twice.  From
> > > kvm_vcpu_arch::regs to tdx_module_args.  From tdx_module_args to real registers.
> > > 
> > > If TDH.VP.ENTER is the only user of __seamcall_saved_ret(), can we make it to
> > > take unsigned long kvm_vcpu_argh::regs[NR_VCPU_REGS]?  Maybe I can make the
> > > change with TDX KVM patch series.
> > 
> > The assembly code assumes the second argument is a pointer to 'struct
> > tdx_module_args'.  I don't know how can we change __seamcall_saved_ret() to
> > achieve what you said.  We might change the kvm_vcpu_argh::regs[NR_VCPU_REGS] to
> > match 'struct tdx_module_args''s layout and manually convert part of "regs" to
> > the structure and pass to __seamcall_saved_ret(), but it's too hacky I suppose.
> 
> I suspect the kvm_vcpu_arch::regs layout is given by hardware; so the
> only option would be to make tdx_module_args match that. It's a slightly
> unfortunate layout, but meh.
> 
> Then you can simply do:
> 
> 	__seamcall_saved_ret(leaf, (struct tdx_module_args *)vcpu->arch->regs);
> 
> 

I don't think the layout matches hardware, especially I think there's no
"hardware layout" for GPRs that are concerned here.  They are just for KVM
itself to save guest's registers when the guest exits to KVM, so that KVM can
restore them when returning back to the guest.