linux-kernel - Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to protect dynamic user state

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Date:   Tue, 31 Aug 2021 18:15:55 -0400
From:   Len Brown <lenb@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Borislav Petkov <bp@...en8.de>,
        "Chang S. Bae" <chang.seok.bae@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, X86 ML <x86@...nel.org>,
        "Brown, Len" <len.brown@...el.com>,
        Thiago Macieira <thiago.macieira@...el.com>,
        "Liu, Jing2" <jing2.liu@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v9 12/26] x86/fpu/xstate: Use feature disable (XFD) to
 protect dynamic user state

On Mon, Aug 30, 2021 at 2:04 PM Dave Hansen <dave.hansen@...el.com> wrote:
>
> On 8/24/21 4:17 PM, Len Brown wrote:
> > Even if your AMX thread pool threads were to invoke this system call
> > as soon as possible...
> > What is to say that the thread pool is created only at a time when memory
> > is available?  A thread could be created 24 hours into program execution
> > under OOM conditions and this system call will return ENOMEM, and your program
> > will in all likelihood throw up its arms and exit at the exact same place
> > it would exit for transparently allocated buffers.
>
> I tried this exact line of reasoning with Thomas: it doesn't matter
> where we run out of memory, we still need the same memory and we're
> screwed either way.
>
> However, Thomas expressed a clear preference for ABIs which return
> memory failures explicitly at syscalls versus implicit failures which
> can happen on random instructions.
>
> One might say that the odds of checking for and handling a NULL value
> (or ENOMEM) are the same as installing a signal handler.  *But*, it's
> infinitely easier to unroll state and recover from a NULL than it is to
> handle it from within a signal handler.  In other words, the explicit
> ones *encourage* better programming.

I agree.
Indeed, I believe that there is universal agreement that a synchronous
return code
from a system call is a far superior programming model than decoding
the location of a failure in a system call.  (no, the IP isn't random -- it is
always the 1st instruction in that thread to touch a TMM register).

> I'd prefer removing the demand-driven allocation at this point.

Adding a pre-allocate system call that can gracefully fail
(even though it never will) is independent from removing
demand-driver allocation.  I would leave this to application
developers.  Honestly, the kernel shouldn't care.

-- 
Len Brown, Intel Open Source Technology Center