Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[H. Peter Anvin]

The recent fix for the AMD FXSAVE information leak had a problematic side effect.  It 
introduced an entry in the x86 features vector which is a bug, not a feature.

The problem with this is that the features vector is designed so that it can be ANDed 
between CPUs to find out the common feature set.  However, bugs aren't features, and bugs 
should be ORd, not ANDed.  In that sense, the *absence* of a bug is a feature.

There are two possible ways of dealing with this:

a) put bugs in the features vector, but sense-inversed, i.e. 1 = bug absent; 0 = bug present.

b) add a separate bugs vector.

When I originally wrote the code I always meant to do (b), but never got around to it. 
It's clear, though, that whatever way we go about this, it should also incorporate the 
fdiv, hlt, f00f, and coma bugs.

	-hpa

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[Andi Kleen]

"H. Peter Anvin" <hpa@zytor.com> writes:

> The recent fix for the AMD FXSAVE information leak had a problematic
> side effect.  It introduced an entry in the x86 features vector which
> is a bug, not a feature.

It's a non issue because it affects all AMD CPUs (except K5/K6).
You'll never find a system where only some CPUs have this problem.

-Andi

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[H. Peter Anvin]

Andi Kleen wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
> 
>> The recent fix for the AMD FXSAVE information leak had a problematic
>> side effect.  It introduced an entry in the x86 features vector which
>> is a bug, not a feature.
> 
> It's a non issue because it affects all AMD CPUs (except K5/K6).
> You'll never find a system where only some CPUs have this problem.
> 

It's still wrong architecturally, and we should have a sane way to deal with this as well 
as other bugs.  This isn't the only bug -- we're getting a decent-size collection of them 
already -- and not all of them is going to have that particular property.

	-hpa

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[Andi Kleen]

On Friday 05 May 2006 20:18, H. Peter Anvin wrote:
> Andi Kleen wrote:
> > "H. Peter Anvin" <hpa@zytor.com> writes:
> > 
> >> The recent fix for the AMD FXSAVE information leak had a problematic
> >> side effect.  It introduced an entry in the x86 features vector which
> >> is a bug, not a feature.
> > 
> > It's a non issue because it affects all AMD CPUs (except K5/K6).
> > You'll never find a system where only some CPUs have this problem.
> > 
> 
> It's still wrong architecturally, and we should have a sane way to deal with this as well 
> as other bugs.  This isn't the only bug -- we're getting a decent-size collection of them 
> already -- and not all of them is going to have that particular property.

I'm not very worried because mixing steppings is generally unsupported anyways
and in Linux also doesn't work well with it.

(e.g. my old rig with one CPU FXSAVE capable, one CPU not never worked without
kernel hacks) 

It's probably not worth the effort anyways. The people who build such strange
systems can tweak their kernels for it too.

-Andi

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[Christer Weinigel]

Andi Kleen <ak@suse.de> writes:

> "H. Peter Anvin" <hpa@zytor.com> writes:
> 
> > The recent fix for the AMD FXSAVE information leak had a problematic
> > side effect.  It introduced an entry in the x86 features vector which
> > is a bug, not a feature.
> 
> It's a non issue because it affects all AMD CPUs (except K5/K6).
> You'll never find a system where only some CPUs have this problem.

I have a dual CPU system (Tyan Tomcat 1564D) where only one CPU is
reported to have the F00F bug (iirc).  So yes, there can be
SMP-systems where the CPU's have different bugs.

    /Christer

-- 
"Just how much can I get away with and still go to heaven?"

Freelance consultant specializing in device driver programming for Linux 
Christer Weinigel <christer@weinigel.se>  http://www.weinigel.se

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[Robert Hancock]

Christer Weinigel wrote:
> Andi Kleen <ak@suse.de> writes:
> 
>> "H. Peter Anvin" <hpa@zytor.com> writes:
>>
>>> The recent fix for the AMD FXSAVE information leak had a problematic
>>> side effect.  It introduced an entry in the x86 features vector which
>>> is a bug, not a feature.
>> It's a non issue because it affects all AMD CPUs (except K5/K6).
>> You'll never find a system where only some CPUs have this problem.
> 
> I have a dual CPU system (Tyan Tomcat 1564D) where only one CPU is
> reported to have the F00F bug (iirc).  So yes, there can be
> SMP-systems where the CPU's have different bugs.

Point being, not this bug..

-- 
Robert Hancock      Saskatoon, SK, Canada
To email, remove "nospam" from hancockr@nospamshaw.ca
Home Page: http://www.roberthancock.com/

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[H. Peter Anvin]

Followup to:  <445CDED0.4070402@shaw.ca>
By author:    Robert Hancock <hancockr@shaw.ca>
In newsgroup: linux.dev.kernel
> > 
> > I have a dual CPU system (Tyan Tomcat 1564D) where only one CPU is
> > reported to have the F00F bug (iirc).  So yes, there can be
> > SMP-systems where the CPU's have different bugs.
> 
> Point being, not this bug..
> 

However, *MY* point is that we should have a uniform infrastructure
for bugs, just like we now have for features.

	-hpa

Re: Bugs aren't features: X86_FEATURE_FXSAVE_LEAK

[Bill Davidsen]

Andi Kleen wrote:
> "H. Peter Anvin" <hpa@zytor.com> writes:
> 
>> The recent fix for the AMD FXSAVE information leak had a problematic
>> side effect.  It introduced an entry in the x86 features vector which
>> is a bug, not a feature.
> 
> It's a non issue because it affects all AMD CPUs (except K5/K6).
> You'll never find a system where only some CPUs have this problem.
> 
The initial argument is dead on, bugs should be either presented as a
"fixed" feature, or there could (should?) be a bugs vector which could
be or'd. It's at least possible that AMD might fix this some time.

-- 
    -bill davidsen (davidsen@tmr.com)
"The secret to procrastination is to put things off until the
  last possible moment - but no longer"  -me