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Message-ID: <5cc5ba09636647a076206fae932bbf88f233b8b2.camel@intel.com>
Date: Thu, 13 Jul 2023 10:19:49 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "peterz@...radead.org" <peterz@...radead.org>
CC: "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>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"pbonzini@...hat.com" <pbonzini@...hat.com>,
"Yamahata, Isaku" <isaku.yamahata@...el.com>,
"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
"sathyanarayanan.kuppuswamy@...ux.intel.com"
<sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH 07/10] x86/tdx: Extend TDX_MODULE_CALL to support more
TDCALL/SEAMCALL leafs
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 :-(
>
> > 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.
>
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?
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