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Re: [Xen-devel] [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware



On 04/25/2012 11:05 AM, Dan Magenheimer wrote:
From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
Subject: RE: [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is 
supported by hardware

On 25.04.12 at 17:01, Dan Magenheimer<dan.magenheimer@xxxxxxxxxx>  wrote:
  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
Sent: Wednesday, April 25, 2012 3:27 AM
To: Boris Ostrovsky; Dan Magenheimer
Cc: wei.huang2@xxxxxxx; xen-devel; keir@xxxxxxx
Subject: Re: [PATCH] svm: Do not intercept RDTCS(P) when TSC scaling is
supported by hardware
On 20.04.12 at 04:21, Boris Ostrovsky<boris.ostrovsky@xxxxxxx>  wrote:
# HG changeset patch
# User Boris Ostrovsky<boris.ostrovsky@xxxxxxx>
# Date 1334875170 14400
# Node ID 55bf11ebce87ceb73fb2c372dcef170ec0bb4a18
# Parent  7c777cb8f705411b77c551f34ba88bdc09e38ab8
svm: Do not intercept RDTCS(P) when TSC scaling is supported by hardware

When running in TSC_MODE_ALWAYS_EMULATE mode on processors that support
TSC scaling we don't need to intercept RDTSC/RDTSCP instructions.

Signed-off-by: Boris Ostrovsky<boris.ostrovsky@xxxxxxx>
Acked-by: Wei Huang<wei.huang2@xxxxxxx>
Tested-by: Wei Huang<wei.huang2@xxxxxxx>
So what's the status of the discussion around this patch? Were
your concerns all addressed, Dan? Is there any re-submisson
necessary/planned?
My concerns will be addressed when there is a fully-functional
adequately-tested full-stack implementation, rather than "we
have a new instruction that should solve (part of) this problem,
let's turn it on by default."

While I wish I could invest the time required to do (or
participate in) the testing, sadly I can't, so I understand
if my opinion is discarded.
As Keir had asked to get an ACK/NAK from you - is this then a NAK
or a "don't care" or yet something else (it doesn't read anywhere
close to an ACK in any case).
Something else ;-)

I certainly don't feel comfortable ACKing it.  I'd like
to see some testing that demonstrates the patch either improves
functionality or performance without breaking other things.
But if nobody else shares my concern, I don't feel that
I have the right to block it either.

We can provide some rdtsc performance numbers. Regarding functionality, it is relatively hard to prove unless Dan has some more specific ideas of testing it. I think the hardware rdtsc scaling is inline with software-based emulated approach.

-Wei




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