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Date:   Thu, 13 Jul 2023 10:43:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     "Huang, Kai" <kai.huang@intel.com>
Cc:     "Hansen, Dave" <dave.hansen@intel.com>,
        "Christopherson,, Sean" <seanjc@google.com>,
        "bp@alien8.de" <bp@alien8.de>, "x86@kernel.org" <x86@kernel.org>,
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        "mingo@redhat.com" <mingo@redhat.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
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        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "Yamahata, Isaku" <isaku.yamahata@intel.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "sathyanarayanan.kuppuswamy@linux.intel.com" 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
 TDCALL/SEAMCALL leafs
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On Thu, Jul 13, 2023 at 08:02:54AM +0000, Huang, Kai wrote:

> Sorry I am ignorant here.  Won't "clearing ECX only" leave high bits of
> registers still containing guest's value?

architecture zero-extends 32bit stores

> I see KVM code uses:
> 
>         xor %eax, %eax
>         xor %ecx, %ecx
>         xor %edx, %edx
>         xor %ebp, %ebp
>         xor %esi, %esi
>         xor %edi, %edi
> #ifdef CONFIG_X86_64
>         xor %r8d,  %r8d
>         xor %r9d,  %r9d
>         xor %r10d, %r10d
>         xor %r11d, %r11d
>         xor %r12d, %r12d
>         xor %r13d, %r13d
>         xor %r14d, %r14d
>         xor %r15d, %r15d
> #endif
> 
> Which makes sense because KVM wants to support 32-bit too.

Encoding for the first lot is shorter, the 64bit regs obviously need the
RAX byte anyway.

> However for TDX is 64-bit only.
> 
> And I also see the current TDVMCALL code has:
> 
>         xor %r8d,  %r8d
>         xor %r9d,  %r9d
>         xor %r10d, %r10d                                                       
>         xor %r11d, %r11d                                                       
>         xor %rdi,  %rdi                                                        
>         xor %rdx,  %rdx
> 
> Why does it need to use "d" postfix for all r* registers?

That's the name of the 32bit subword, r#[bwd] for byte, word,
double-word. SDM v1 3.7.2.1 has the whole list, I couldn't quicky find
one for the zero-extention thing.

> Sorry for those questions but I struggled when I wrote those assembly and am
> hoping to get my mind cleared on this. :-)

No problem.