linux-kernel - Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more TDCALL/SEAMCALL leafs

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Date:   Thu, 13 Jul 2023 11:40:10 +0000
From:   "Huang, Kai" <kai.huang@...el.com>
To:     "andrew.cooper3@...rix.com" <andrew.cooper3@...rix.com>
CC:     "Hansen, Dave" <dave.hansen@...el.com>,
        "Christopherson,, Sean" <seanjc@...gle.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "hpa@...or.com" <hpa@...or.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
 TDCALL/SEAMCALL leafs

On Thu, 2023-07-13 at 12:22 +0100, Andrew Cooper wrote:
> On Thu, 13 Jul 2023 10:47:44 +0000, Huang, Kai wrote:
> > On Thu, 2023-07-13 at 12:37 +0200, Peter Zijlstra wrote:
> > > 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.
> > 
> > Learned something new.  Appreciate your time! :-)
> 
> And now for the extra fun...
> 
> The Silvermont uarch is 64bit, but only recognises 32bit XORs as zeroing
> idioms.
> 
> So for best performance on as many uarches as possible, you should *always*
> use the 32bit forms, even for %r8-15.
> 
> 

Ah thanks Andrew for the tip :)