Messages in this thread | | | Subject | Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core() implementation | From | Boris Ostrovsky <> | Date | Fri, 2 Dec 2016 15:09:31 -0500 |
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On 12/02/2016 06:44 AM, Andrew Cooper 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. > >> 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@citrix.com> >> Signed-off-by: Andy Lutomirski <luto@kernel.org>
Executing CPUID in an HVM guest is quite expensive since it will cause a VMEXIT. (And that should be true for any hypervisor, at least on Intel. On AMD it's configurable)
-boris
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