linux-kernel - Re: [Xen-devel] [PATCHv1] x86: don't schedule when handling #NM exception

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Mon, 17 Mar 2014 09:55:57 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	George Dunlap <George.Dunlap@...citrix.com>
CC:	Sarah Newman <srn@...mr.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>
Subject: Re: [Xen-devel] [PATCHv1] x86: don't schedule when handling #NM exception

On 03/17/2014 05:19 AM, George Dunlap wrote:
> On Mon, Mar 17, 2014 at 3:33 AM, H. Peter Anvin <hpa@...or.com> wrote:
>> No, the right thing is to unf*ck the Xen braindamage and use eagerfpu as a workaround for the legacy hypervisor versions.
> 
> The interface wasn't an accident.  In the most common case you'll want
> to clear the bit anyway. In PV mode clearing it would require an extra
> trip up into the hypervisor.  So this saves one trip up into the
> hypervisor on every context switch which involves an FPU, at the
> expense of not being able to context-switch away when handling the
> trap.
> 
>  -George
> 

The interface was a complete faceplant, because it caused failures.
You're not infinitely unconstrained since you want to play in the same
sandbox as the native architecture, and if you want to have a hope of
avoiding these kinds of failures you really need to avoid making random
"improvements", certainly not without an explicit guest opt-in (the same
we do for the native CPU architecture when adding new features.)

So if this interface wasn't an accident it was active negligence and
incompetence.

	-hpa

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/