From mboxrd@z Thu Jan  1 00:00:00 1970
From: Paolo Bonzini <pbonzini@redhat.com>
Subject: Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed
 up the vdso pvclock reader
Date: Tue, 23 Dec 2014 16:14:11 +0100
Message-ID: <549986C3.4060807@redhat.com>
References: <cover.1419295081.git.luto@amacapital.net> <8d09c16eb39cbe264417cc66c4aca730af10b70b.1419295081.git.luto@amacapital.net> <549986B8.3030606@oracle.com>
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Cc: Gleb Natapov <gleb@kernel.org>,
	"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	kvm list <kvm@vger.kernel.org>
To: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Andy Lutomirski <luto@amacapital.net>,
	Marcelo Tosatti <mtosatti@redhat.com>
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On 23/12/2014 16:14, Boris Ostrovsky wrote:
>> +    do {
>> +        version = pvti->version;
>> +
>> +        /* This is also a read barrier, so we'll read version first. */
>> +        rdtsc_barrier();
>> +        tsc = __native_read_tsc();
> 
> 
> This will cause VMEXIT on Xen with TSC_MODE_ALWAYS_EMULATE which is
> used, for example, after guest migrated (unless HW is capable of scaling
> TSC rate).

So does the __pvclock_read_cycles this is replacing (via
pvclock_get_nsec_offset).

Paolo