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Message-ID: <1d68c6dd-df69-ffc0-bb36-10b4b76bb2fb@oracle.com>
Date: Fri, 2 Dec 2016 15:09:31 -0500
From: Boris Ostrovsky <boris.ostrovsky@...cle.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>,
Andy Lutomirski <luto@...nel.org>, x86@...nel.org
Cc: One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
Borislav Petkov <bp@...en8.de>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
Brian Gerst <brgerst@...il.com>,
Matthew Whitehead <tedheadster@...il.com>,
Henrique de Moraes Holschuh <hmh@....eng.br>,
Peter Zijlstra <peterz@...radead.org>,
Xen-devel List <xen-devel@...ts.xen.org>,
Juergen Gross <JGross@...e.com>
Subject: Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync_core()
implementation
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@...rix.com>
>> Signed-off-by: Andy Lutomirski <luto@...nel.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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