linux-kernel - Re: [PATCH 1/2] x86/kexec: Preserve CR4.MCE during kexec

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Message-ID: <20230216094340.bl3k5q6ine2luhoz@box.shutemov.name>
Date:   Thu, 16 Feb 2023 12:43:40 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc:     "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "Hansen, Dave" <dave.hansen@...el.com>,
        "sathyanarayanan.kuppuswamy@...ux.intel.com" 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
        "Yamahata, Isaku" <isaku.yamahata@...el.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [PATCH 1/2] x86/kexec: Preserve CR4.MCE during kexec

On Thu, Feb 16, 2023 at 01:49:39AM +0000, Edgecombe, Rick P wrote:
> On Tue, 2023-02-14 at 02:48 +0300, Kirill A. Shutemov wrote:
> > TDX guests are not allowed to clear CR4.MCE. Attempt to clear it
> > leads
> > to #VE.
> > 
> > Preserve the flag during kexec.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> 
> I wonder whats going on with the pre-existing switching between eax and
> rax in this code for the cr0 and cr4 manipulations. Do you know what
> the reason is?

32-bit ORs and ANDs save one byte per instruction. And there's no 32-bit
MOV to/from control registers in 64-bit mode.

> 
> Also, for a simple non-tdx kexec regression test:
> Tested-by: Rick Edgecombe <rick.p.edgecombe@...el.com>

Thanks!

-- 
  Kiryl Shutsemau / Kirill A. Shutemov