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

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Message-ID: <20130604145304.117ac60f@mantra.us.oracle.com>
Date:	Tue, 4 Jun 2013 14:53:04 -0700
From:	Mukesh Rathor <mukesh.rathor@...cle.com>
To:	"Jan Beulich" <JBeulich@...e.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 Tue, 04 Jun 2013 09:27:03 +0100
"Jan Beulich" <JBeulich@...e.com> wrote:

> >>> On 04.06.13 at 02:43, Mukesh Rathor <mukesh.rathor@...cle.com>
> >>> wrote:
> > @@ -1327,6 +1329,18 @@ static void __init
> > xen_setup_stackprotector(void) /* PVH TBD/FIXME: investigate
> > setup_stack_canary_segment */ if
> > (xen_feature(XENFEAT_auto_translated_physmap))
> > { switch_to_new_gdt(0); +
> > +		/* xen started us with null selectors. load them
> > now */
> > +		__asm__ __volatile__ (
> > +			"movl %0,%%ds\n"
> > +			"movl %0,%%ss\n"
> > +			"pushq %%rax\n"
> > +			"leaq 1f(%%rip),%%rax\n"
> > +			"pushq %%rax\n"
> > +			"retfq\n"
> > +			"1:\n"
> > +			: : "r" (__KERNEL_DS), "a" (__KERNEL_CS) :
> > "memory"); +
> 
> I can see why you want CS to be reloaded (and CS, other than the
> comment says, clearly hasn't been holding a null selector up to here.
> 
> I can't immediately see why you'd need SS to be other than null, and
> it completely escapes me why you'd need to DS (but not ES) to be
> non-null.
> 
> Furthermore, is there any reason why you use "retfq" (Intel syntax)
> when all assembly code otherwise uses AT&T syntax (the proper
> equivalent here would be "lretq")?
> 
> And finally, please consistently use %<number> (which, once
> fixed, will make clear that the second constraint really can be "r"),
> and avoid using suffixes on moves to/from selector registers
> (which, once fixed, will make clear that at least the first constraint
> really can be relaxed to "rm").

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;
        }

thanks,
Mukesh

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