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Date:   Thu, 13 Jul 2023 12:37:33 +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>,
        "hpa@zytor.com" <hpa@zytor.com>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "kirill.shutemov@linux.intel.com" <kirill.shutemov@linux.intel.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "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 10:19:49AM +0000, Huang, Kai wrote:
> On Thu, 2023-07-13 at 10:43 +0200, Peter Zijlstra wrote:
> > 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
> 
> Sorry, where can I find this information? Looking at SDM I couldn't find :-(

Yeah, I couldn't find it in a hurry either, but bpetkov pasted me this
from the AMD document:

 "In 64-bit mode, the following general rules apply to instructions and their operands:
 “Promoted to 64 Bit”: If an instruction’s operand size (16-bit or 32-bit) in legacy and
 compatibility modes depends on the CS.D bit and the operand-size override prefix, then the
 operand-size choices in 64-bit mode are extended from 16-bit and 32-bit to include 64 bits (with a
 REX prefix), or the operand size is fixed at 64 bits. Such instructions are said to be “Promoted to
 64 bits” in Table B-1. However, byte-operand opcodes of such instructions are not promoted."

> I _think_ I understand now? In 64-bit mode
> 
> 	xor %eax, %eax
> 
> equals to
> 
> 	xor %rax, %rax
> 
> (due to "architecture zero-extends 32bit stores")
> 
> Thus using the former (plus using "d" for %r*) can save some memory?

Yes, 64bit wide instruction get a REX prefix 0x4X (somehow I keep typing
RAX) byte in front to tell it's a 64bit wide op.

   31 c0                   xor    %eax,%eax
   48 31 c0                xor    %rax,%rax

The REX byte will show up for rN usage, because then we need the actual
Register Extention part of that prefix irrespective of the width.

   45 31 d2                xor    %r10d,%r10d
   4d 31 d2                xor    %r10,%r10

x86 instruction encoding is 'fun' :-)

See SDM Vol 2 2.2.1.2 if you want to know more about the REX prefix.