Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation

[Andy Lutomirski <luto@amacapital.net>]

On Fri, Dec 2, 2016 at 11:24 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Fri, Dec 2, 2016 at 11:20 AM, Borislav Petkov <bp@alien8.de> wrote:
>>
>> Something like below?
>
> The optimize-nops thing needs it too, I think.
>
> Again, this will never matter in practice (even if somebody has a i486
> s till, the prefetch window size is like 16 bytes or something), but
> from a documentation standpoint it's good.

How's this?

/*
 * This function forces the icache and prefetched instruction stream to
 * catch up with reality in two very specific cases:
 *
 *  a) Text was modified using one virtual address and is about to be executed
 *     from the same physical page at a different virtual address.
 *
 *  b) Text was modified on a different CPU, may subsequently be
 *     executed on this CPU, and you want to make sure the new version
 *     gets executed.  This generally means you're calling this in a IPI.
 *
 * If you're calling this for a different reason, you're probably doing
 * it wrong.
 */
static inline void native_sync_core(void) { ... }

The body will do a MOV-to-CR2 followed by jmp 1f; 1:.  This sequence
should be guaranteed to flush the pipeline on any real CPU.  On Xen it
will do IRET-to-self.

I suppose it could be an unconditional IRET-to-self, but that's a good
deal slower and not a whole lot simpler.  Although if we start doing
it right, performance won't really matter here.

--Andy