xsaves support broken?

xsaves support broken?

[Dave Hansen]

Hi Fenghua,

I was testing some new MPX code, and realized that MPX had stopped
working completely.  The prctl() failed to turn it on, which I tracked
down to a 0 in "BNDCSR" where we expected to see an enable bit.

I realized that I had managed to remove the "noxsaves" boot option from
my kernel.  Re-adding "noxsaves" made MPX work again, and the BNDCSR
contents were properly passed in to the kernel.

Is it possible that  "xsaves" use inside the kernel is simply broken?

Config is here:

	http://sr71.net/~dave/intel/config-20150302

RE: xsaves support broken?

[Yu, Fenghua]

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> From: Hansen, Dave
> Sent: Monday, March 02, 2015 4:00 PM
> 
> Hi Fenghua,
> 
> I was testing some new MPX code, and realized that MPX had stopped
> working completely.  The prctl() failed to turn it on, which I tracked down to a
> 0 in "BNDCSR" where we expected to see an enable bit.
> 
> I realized that I had managed to remove the "noxsaves" boot option from my
> kernel.  Re-adding "noxsaves" made MPX work again, and the BNDCSR
> contents were properly passed in to the kernel.
> 
> Is it possible that  "xsaves" use inside the kernel is simply broken?
> 
> Config is here:
> 
> 	http://sr71.net/~dave/intel/config-20150302

Xsaves has been tested by QA since 3.17 with or without MPX. I'm not aware of reported issues.

But MPX is the only code to call the get_xsave_addr kernel API. Though it has been tested before on upstream MPX code.

What changes in your new MPX code?

Thanks.

-Fenghua
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Re: xsaves support broken?

[Dave Hansen]

On 03/02/2015 04:09 PM, Yu, Fenghua wrote:
> Xsaves has been tested by QA since 3.17 with or without MPX. I'm not
> aware of reported issues.
> 
> But MPX is the only code to call the get_xsave_addr kernel API.
> Though it has been tested before on upstream MPX code.

Has it?  I'm fairly sure I've never explicitly tested it.  I'm also
doubting now that I've *ever* seen it work without booting with 'noxsaves'.

> What changes in your new MPX code?

I just discovered it when adding a new feature.  I actually backed out
all of my changes before reporting this issue.  I'm running a completely
untouched 4.0-rc1 kernel with the config I attached a moment ago.

If I boot with 'noxsaves', MPX works fine.  If I boot without it, the
kernel never sees the values that userspace sets in the "bndcsr" register.

RE: xsaves support broken?

[Yu, Fenghua]

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> From: Hansen, Dave
> On 03/02/2015 04:09 PM, Yu, Fenghua wrote:
> > Xsaves has been tested by QA since 3.17 with or without MPX. I'm not
> > aware of reported issues.
> >
> > But MPX is the only code to call the get_xsave_addr kernel API.
> > Though it has been tested before on upstream MPX code.
> 
> Has it?  I'm fairly sure I've never explicitly tested it.  I'm also doubting now
> that I've *ever* seen it work without booting with 'noxsaves'.

QA did test conflict between MPX and xsaves long time before MPX went upstream. But seems the conflict happened again at some point...

> 
> > What changes in your new MPX code?
> 
> I just discovered it when adding a new feature.  I actually backed out all of my
> changes before reporting this issue.  I'm running a completely untouched 4.0-
> rc1 kernel with the config I attached a moment ago.

MPX and xsaves are supposed to work well on 4.0-rc1. If not, definitely the code is broken again somehow.

> If I boot with 'noxsaves', MPX works fine.  If I boot without it, the kernel
> never sees the values that userspace sets in the "bndcsr" register.

We are supposed to handle this issue long time ago.

I can try to reproduce the issue with the config.

Thanks.

-Fenghua

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