linux-kernel - Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT

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Date:   Mon, 30 Apr 2018 17:57:04 +0100
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Boris Ostrovsky <boris.ostrovsky@...cle.com>
CC:     <linux-kernel@...r.kernel.org>, <xen-devel@...ts.xenproject.org>,
        <jgross@...e.com>, <stable@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH 1/4] xen/PVH: Replace GDT_ENTRY with explicit
 constant

On Mon, Apr 30, 2018 at 12:23:36PM -0400, Boris Ostrovsky wrote:
> Latest binutils release (2.29.1) will no longer allow proper computation
> of GDT entries on 32-bits, with warning:
> 
> arch/x86/xen/xen-pvh.S: Assembler messages:
> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31)
> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (40 is not between 0 and 31)
> arch/x86/xen/xen-pvh.S:150: Warning: shift count out of range (32 is not between 0 and 31)
> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31)
> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (40 is not between 0 and 31)
> arch/x86/xen/xen-pvh.S:152: Warning: shift count out of range (32 is not between 0 and 31)
> 
> Use explicit value of the entry instead of using GDT_ENTRY() macro.
> 
> Signed-off-by: Boris Ostrovsky <boris.ostrovsky@...cle.com>
> Cc: stable@...r.kernel.org
> ---
>  arch/x86/xen/xen-pvh.S | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/x86/xen/xen-pvh.S b/arch/x86/xen/xen-pvh.S
> index e1a5fbe..934f7d4 100644
> --- a/arch/x86/xen/xen-pvh.S
> +++ b/arch/x86/xen/xen-pvh.S
> @@ -145,11 +145,11 @@ gdt_start:
>  	.quad 0x0000000000000000            /* NULL descriptor */
>  	.quad 0x0000000000000000            /* reserved */
>  #ifdef CONFIG_X86_64
> -	.quad GDT_ENTRY(0xa09a, 0, 0xfffff) /* __KERNEL_CS */
> +	.quad 0x00af9a000000ffff            /* __BOOT_CS */
>  #else
> -	.quad GDT_ENTRY(0xc09a, 0, 0xfffff) /* __KERNEL_CS */
> +	.quad 0x00cf9a000000ffff            /* __BOOT_CS */

Maybe it would be cleaner to use something like:

.word 0xffff /* limit */
.word 0      /* base */
.byte 0      /* base */
.byte 0x9a   /* access */
#ifdef CONFIG_X86_64
.byte 0xaf   /* flags plus limit */
#else
.byte 0xcf   /* flags plus limit */
#endif
.byte 0      /* base */

Or try to fix the GDT_ENTRY macro, maybe if you prepend extra 0's to
make the values 64bit that would prevent the warnings?

Or declare the GDT in enlighten_pvh in C and use it here?

Also, IIRC the base/limit values are ignored in long mode.

Thanks, Roger.