Messages in this thread | | | Date | Wed, 05 Jun 2013 08:03:12 +0100 | From | "Jan Beulich" <> | Subject | Re: [Xen-devel] [PATCH] PVH: vcpu info placement, load selectors, and remove debug printk. |
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>>> On 04.06.13 at 23:53, Mukesh Rathor <mukesh.rathor@oracle.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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