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From: Peter Zijlstra <peterz@infradead.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>,
	Weiny Ira <ira.weiny@intel.com>, Ingo Molnar <mingo@redhat.com>,
	Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Fenghua Yu <fenghua.yu@intel.com>,
	"open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
	linux-nvdimm <linux-nvdimm@lists.01.org>,
	Linux-MM <linux-mm@kvack.org>,
	"open list:KERNEL SELFTEST FRAMEWORK" 
	<linux-kselftest@vger.kernel.org>,
	Dan Williams <dan.j.williams@intel.com>,
	Greg KH <gregkh@linuxfoundation.org>
Subject: Re: [PATCH V3.1] entry: Pass irqentry_state_t by reference
Date: Thu, 17 Dec 2020 14:19:24 +0100	[thread overview]
Message-ID: <20201217131924.GW3040@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <878s9wshsa.fsf@nanos.tec.linutronix.de>

On Thu, Dec 17, 2020 at 02:07:01PM +0100, Thomas Gleixner wrote:
> On Fri, Dec 11 2020 at 14:14, Andy Lutomirski wrote:
> > On Mon, Nov 23, 2020 at 10:10 PM <ira.weiny@intel.com> wrote:
> > After contemplating this for a bit, I think this isn't really the
> > right approach.  It *works*, but we've mostly just created a bit of an
> > unfortunate situation.  Our stack, on a (possibly nested) entry looks
> > like:
> >
> > previous frame (or empty if we came from usermode)
> > ---
> > SS
> > RSP
> > FLAGS
> > CS
> > RIP
> > rest of pt_regs
> >
> > C frame
> >
> > irqentry_state_t (maybe -- the compiler is within its rights to play
> > almost arbitrary games here)
> >
> > more C stuff
> >
> > So what we've accomplished is having two distinct arch register
> > regions, one called pt_regs and the other stuck in irqentry_state_t.
> > This is annoying because it means that, if we want to access this
> > thing without passing a pointer around or access it at all from outer
> > frames, we need to do something terrible with the unwinder, and we
> > don't want to go there.
> >
> > So I propose a somewhat different solution: lay out the stack like this.
> >
> > SS
> > RSP
> > FLAGS
> > CS
> > RIP
> > rest of pt_regs
> > PKS
> > ^^^^^^^^ extended_pt_regs points here
> >
> > C frame
> > more C stuff
> > ...
> >
> > IOW we have:
> >
> > struct extended_pt_regs {
> >   bool rcu_whatever;
> >   other generic fields here;
> >   struct arch_extended_pt_regs arch_regs;
> >   struct pt_regs regs;
> > };
> >
> > and arch_extended_pt_regs has unsigned long pks;
> >
> > and instead of passing a pointer to irqentry_state_t to the generic
> > entry/exit code, we just pass a pt_regs pointer.
> 
> While I agree vs. PKS which is architecture specific state and needed in
> other places e.g. #PF, I'm not convinced that sticking the existing
> state into the same area buys us anything more than an indirect access.
> 
> Peter?

Agreed; that immediately solves the confusion Ira had as well. While
extending pt_regs sounds scary, I think we've isolated our pt_regs
implementation from actual ABI pretty well, but of course, that would
need an audit. We don't want to leak this into signals for example.



  reply	other threads:[~2020-12-17 13:20 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-11-06 23:28 [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 ira.weiny
2020-11-06 23:28 ` [PATCH V3 01/10] x86/pkeys: Create pkeys_common.h ira.weiny
2020-11-06 23:29 ` [PATCH V3 02/10] x86/fpu: Refactor arch_set_user_pkey_access() for PKS support ira.weiny
2020-11-06 23:29 ` [PATCH V3 03/10] x86/pks: Add PKS defines and Kconfig options ira.weiny
2020-11-06 23:29 ` [PATCH V3 04/10] x86/pks: Preserve the PKRS MSR on context switch ira.weiny
2020-12-17 14:50   ` Thomas Gleixner
2020-12-17 22:43     ` Thomas Gleixner
2020-12-18 13:57       ` Thomas Gleixner
2020-12-18 19:20         ` Dan Williams
2020-12-18 21:06           ` Thomas Gleixner
2020-12-18 21:58             ` Dan Williams
2020-12-18 22:44               ` Thomas Gleixner
2020-12-18 19:42         ` Ira Weiny
2020-12-18 20:10           ` Dave Hansen
2020-12-18 21:30           ` Thomas Gleixner
2020-12-18  4:05     ` Ira Weiny
2020-12-17 20:41   ` [NEEDS-REVIEW] " Dave Hansen
2020-12-18  4:10     ` Ira Weiny
2020-12-18 15:33       ` Dave Hansen
2020-11-06 23:29 ` [PATCH V3 05/10] x86/entry: Pass irqentry_state_t by reference ira.weiny
2020-11-15 18:58   ` Thomas Gleixner
2020-11-16 18:49     ` Ira Weiny
2020-11-16 20:36       ` Thomas Gleixner
2020-11-24  6:09   ` [PATCH V3.1] entry: " ira.weiny
2020-12-11 22:14     ` Andy Lutomirski
2020-12-16  1:32       ` Ira Weiny
2020-12-16  2:09         ` Andy Lutomirski
2020-12-17  0:38           ` Ira Weiny
2020-12-17 13:07       ` Thomas Gleixner
2020-12-17 13:19         ` Peter Zijlstra [this message]
2020-12-17 15:35           ` Andy Lutomirski
2020-12-17 16:58     ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 06/10] x86/entry: Preserve PKRS MSR across exceptions ira.weiny
2020-12-17 15:28   ` Thomas Gleixner
2020-11-06 23:29 ` [PATCH V3 07/10] x86/fault: Report the PKRS state on fault ira.weiny
2020-11-06 23:29 ` [PATCH V3 08/10] x86/pks: Add PKS kernel API ira.weiny
2020-12-23 20:39   ` Randy Dunlap
2020-11-06 23:29 ` [PATCH V3 09/10] x86/pks: Enable Protection Keys Supervisor (PKS) ira.weiny
2020-11-06 23:29 ` [PATCH V3 10/10] x86/pks: Add PKS test code ira.weiny
2020-12-17 20:55   ` Dave Hansen
2020-12-18  4:05     ` Ira Weiny
2020-12-18 16:59       ` Dan Williams
2020-12-07 22:14 ` [PATCH V3 00/10] PKS: Add Protection Keys Supervisor (PKS) support V3 Ira Weiny
2020-12-08 15:55   ` Thomas Gleixner
2020-12-08 17:22     ` Ira Weiny

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