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

[Andrew Cooper]

On 02/12/16 17:07, Andy Lutomirski wrote:
> 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.

All returns to a 64bit PV guest at defined points (hypercall return,
exception entry, etc) are from SYSRET, not IRET.

Talking of, I still have a patch to remove
PARAVIRT_ADJUST_EXCEPTION_FRAME which I need to complete and send upstream.

>
>>> 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.

Fair enough.  (Assuming similar properties hold for the older processors
of other vendors.)

~Andrew