linux-kernel - Re: [Stable-review] [12/28] x86, cpu: Clean up AMD erratum 400 workaround

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Message-ID: <1303265862.3464.77.camel@localhost>
Date:	Wed, 20 Apr 2011 03:17:42 +0100
From:	Ben Hutchings <ben@...adent.org.uk>
To:	Greg KH <gregkh@...e.de>
Cc:	Hans Rosenfeld <hans.rosenfeld@....com>,
	linux-kernel@...r.kernel.org, stable@...nel.org,
	akpm@...ux-foundation.org, "H. Peter Anvin" <hpa@...ux.intel.com>,
	torvalds@...ux-foundation.org, stable-review@...nel.org,
	alan@...rguk.ukuu.org.uk
Subject: Re: [Stable-review] [12/28] x86, cpu: Clean up AMD erratum 400
 workaround

On Tue, 2011-04-19 at 19:01 -0700, Greg KH wrote:
> On Wed, Apr 20, 2011 at 02:40:53AM +0100, Ben Hutchings wrote:
> > On Tue, 2011-04-19 at 13:30 -0700, Greg KH wrote:
> > > 2.6.32-longterm review patch.  If anyone has any objections, please let us know.
> > > 
> > > ------------------
> > > 
> > > From: Hans Rosenfeld <hans.rosenfeld@....com>
> > > 
> > > commit 9d8888c2a214aece2494a49e699a097c2ba9498b upstream.
> > > 
> > > Remove check_c1e_idle() and use the new AMD errata checking framework
> > > instead.
> > 
> > Clean-up patches are generally not candidates for longterm updates.
> 
> This was added because a follow-on patch required it.

Ah yes, 'x86, AMD: Set ARAT feature on AMD processors' is using the same
condition.

Of course, that could have been backported by referring to the function
that this removes, rather than pulling in a load of other changes with
consequent risk of regressions.

> > However, I notice that the range of procesors considered to have erratum
> > 400 was also changed:
> > 
> > [...]
> > > +const int amd_erratum_400[] =
> > > +	AMD_OSVW_ERRATUM(1, AMD_MODEL_RANGE(0xf, 0x41, 0x2, 0xff, 0xf),
> > > +			    AMD_MODEL_RANGE(0x10, 0x2, 0x1, 0xff, 0xf));
> > [...]
> > > -	/* Family 0x0f models < rev F do not have C1E */
> > > -	if (c->x86 == 0x0F && c->x86_model >= 0x40)
> > > -		return 1;
> > > -
> > > -	if (c->x86 == 0x10) {
> > > -		/*
> > > -		 * check OSVW bit for CPUs that are not affected
> > > -		 * by erratum #400
> > > -		 */
> > > -		if (cpu_has(c, X86_FEATURE_OSVW)) {
> > [...]
> > > -		}
> > > -		return 1;
> > [...]
> > 
> > Family 0x0f model 0x40 and model 0x41 stepping 0 and 1 are excluded.
> > Family 0x10 model 0x00, 0x01 and model 0x02 stepping 0 are excluded.
> > Is that the real fix here?
> 
> In this patch, no, it's just infrastructure for a later one.
> And I think the bug you noticed here was also fixed in a later patch in
> the series, right?

I don't know that it is a bug.  But if you're referring to patch 26/28
'x86, cpu: Fix regression in AMD errata checking code', that fixes a bug
in range checking in the generic function but does not revert this
change.

Ben.

-- 
Ben Hutchings
Once a job is fouled up, anything done to improve it makes it worse.

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