linux-kernel - Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed up the vdso pvclock reader

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Message-ID: <549986C3.4060807@redhat.com>
Date:	Tue, 23 Dec 2014 16:14:11 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Andy Lutomirski <luto@...capital.net>,
	Marcelo Tosatti <mtosatti@...hat.com>
CC:	Gleb Natapov <gleb@...nel.org>,
	"xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm list <kvm@...r.kernel.org>
Subject: Re: [Xen-devel] [RFC 2/2] x86, vdso, pvclock: Simplify and speed
 up the vdso pvclock reader



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
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