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

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Date:   Thu, 13 Jul 2023 07:51:30 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Kai Huang <kai.huang@...el.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Dave Hansen <dave.hansen@...el.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>,
        Isaku Yamahata <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, Jul 13, 2023, Kai Huang wrote:
> 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.ï¿½ 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.ï¿½ __seamcall_saved_ret() uses struct tdx_module_arg as input and output.ï¿½ For
> > > > KVM TDH.VP.ENTER case, those arguments are already in unsigned long
> > > > kvm_vcpu_arch::regs[].ï¿½ It's silly to move those values twice.ï¿½ From
> > > > kvm_vcpu_arch::regs to tdx_module_args.ï¿½ 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]?ï¿½ 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.

kvm_vcpu_arch::regs does follow the hardware-defined indices, and that's required
for myriad emulation flows, e.g. saving GPRs into SMRAM, anywhere KVM gets a GPR
index from an instruction opcode or vmcs.VMX_INSTRUCTION_INFO, etc.