linux-kernel - Re: [Xen-devel] [PATCH] PVH: vcpu info placement, load selectors, and remove debug printk.

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Message-Id: <51AEFED002000078000DB542@nat28.tlf.novell.com>
Date:	Wed, 05 Jun 2013 08:03:12 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Mukesh Rathor" <mukesh.rathor@...cle.com>
Cc:	"xen-devel" <xen-devel@...ts.xen.org>, <konrad.wilk@...cle.com>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] PVH: vcpu info placement, load
 selectors, and remove debug printk.

>>> On 04.06.13 at 23:53, Mukesh Rathor <mukesh.rathor@...cle.com> wrote:
> Following OK? :
> 
>         if (xen_feature(XENFEAT_auto_translated_physmap)) {
>                 switch_to_new_gdt(0);
> 
>                 asm volatile (
>                         "pushq %%rax\n"
>                         "leaq 1f(%%rip),%%rax\n"
>                         "pushq %%rax\n"
>                         "lretq\n"
>                         "1:\n"
>                         : : "a" (__KERNEL_CS) : "memory");
> 
>                 return;
>         }

While generally the choice of using %%rax instead of %0 here is
a matter of taste to some degree, I still don't see why you can't
use "r" as the constraint here in the first place.

Furthermore, assuming this sits in a function guaranteed to not be
inlined, this has a latent bug (and if the assumption isn't right, the
bug is real) in that the asm() modifies %rax without telling the
compiler.

This is how I would have done it:

		unsigned long dummy;

		asm volatile ("pushq %0\n"
			      "leaq 1f(%%rip),%0\n"
			      "pushq %0\n"
			      "lretq\n"
			      "1:\n"
			      : "=&r" (dummy) : "0" (__KERNEL_CS));

(also dropping the memory clobber, as I don't see what you
need this for).

Jan

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