linux-kernel - Re: [PATCH v2 5/6] x86/xen: Add a Xen-specific sync

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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