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

[Andy Lutomirski]

On Dec 2, 2016 3:44 AM, "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 02/12/16 00:35, Andy Lutomirski wrote:
> > On Xen PV, CPUID is likely to trap, and Xen hypercalls aren't
> > guaranteed to serialize.  (Even CPUID isn't *really* guaranteed to
> > serialize on Xen PV, but, in practice, any trap it generates will
> > serialize.)
>
> Well, Xen will enabled CPUID Faulting wherever it can, which is
> realistically all IvyBridge hardware and newer.
>
> All hypercalls are a privilege change to cpl0.  I'd hope this condition
> is serialising, but I can't actually find any documentation proving or
> disproving this.

I don't know for sure.  IRET is serializing, and if Xen returns using
IRET, we're fine.

>
> >
> > On my laptop, CPUID(eax=1, ecx=0) is ~83ns and IRET-to-self is
> > ~110ns.  But Xen PV will trap CPUID if possible, so IRET-to-self
> > should end up being a nice speedup.
> >
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
>
> CC'ing xen-devel and the Xen maintainers in Linux.
>
> As this is the only email from this series in my inbox, I will say this
> here, but it should really be against patch 6.
>
> A write to %cr2 is apparently (http://sandpile.org/x86/coherent.htm) not
> serialising on the 486, but I don't have a manual to hand to check.

I'll quote the (modern) SDM.  For self-modifying code "The use of one
of these options is not required for programs intended to run on the
Pentium or Intel486 processors,
but are recommended to ensure compatibility with the P6 and more
recent processor families.".  For cross-modifying code "The use of
this option is not required for programs intended to run on the
Intel486 processor, but is recommended
to ensure compatibility with the Pentium 4, Intel Xeon, P6 family, and
Pentium processors."  So I'm not sure there's a problem.

I can add an unconditional jump just to make sure.  It costs basically
nothing on modern CPUs.  (Also, CPUID also isn't serializing on 486
according to the table.)

--Andy