Messages in this thread | | | Date | Thu, 6 Aug 2020 16:04:55 -0700 | From | Ricardo Neri <> | Subject | Re: [PATCH v3] x86/cpu: Use SERIALIZE in sync_core() when available |
| |
On Thu, Aug 06, 2020 at 12:57:26PM -0700, Dave Hansen wrote: > On 8/6/20 12:25 PM, Ricardo Neri wrote: > > static inline void sync_core(void) > > { > > /* > > - * There are quite a few ways to do this. IRET-to-self is nice > > + * Hardware can do this for us if SERIALIZE is available. Otherwise, > > + * there are quite a few ways to do this. IRET-to-self is nice > > This seems to imply that things other than SERIALIZE aren't the hardware > doing this. All of these methods are equally architecturally > serializing *and* provided by the hardware.
Indeed, I can see how the wording may imply that.
> > I also don't quite get the point of separating the comments from the > code. Shouldn't this be: > > /* > * The SERIALIZE instruction is the most straightforward way to > * do this but it not universally available. > */
I regard the comment as describing all the considered options to for architectural serialization. What about if I add SERIALIZE as another option? I propose to discuss it towards the end of the comment:
/* * There are quite a few ways to do this. IRET-to-self is nice * because it works on every CPU, at any CPL (so it's compatible * with paravirtualization), and it never exits to a hypervisor. * The only down sides are that it's a bit slow (it seems to be * a bit more than 2x slower than the fastest options) and that * it unmasks NMIs. The "push %cs" is needed because, in * paravirtual environments, __KERNEL_CS may not be a valid CS * value when we do IRET directly. * * In case NMI unmasking or performance ever becomes a problem, * the next best option appears to be MOV-to-CR2 and an * unconditional jump. That sequence also works on all CPUs, * but it will fault at CPL3 (i.e. Xen PV). * * CPUID is the conventional way, but it's nasty: it doesn't * exist on some 486-like CPUs, and it usually exits to a * hypervisor. * * The SERIALIZE instruction is the most straightforward way to * do this as it does not clobber registers or exit to a * hypervisor. However, it is not universally available. * * Like all of Linux's memory ordering operations, this is a * compiler barrier as well. */
What do you think?
> if (static_cpu_has(X86_FEATURE_SERIALIZE)) { > asm volatile(__ASM_SERIALIZE ::: "memory"); > return; > } > > /* > * For all other processors, IRET-to-self is nice ... > */ > iret_to_self();
| |