From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>,
LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>
Subject: Re: [RFC][PATCH 0/4] x86/entry: disallow #DB more
Date: Sat, 23 May 2020 14:59:40 +0200 [thread overview]
Message-ID: <20200523125940.GA2483@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CALCETrV7GYg5V5dgM9BToc6RAqpcjRdoZoeXbnrTKTqjBfft6g@mail.gmail.com>
On Fri, May 22, 2020 at 03:13:57PM -0700, Andy Lutomirski wrote:
> On Fri, May 22, 2020 at 1:49 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > Hai, this kills #DB during NMI/#MC and with that allows removing all the nasty
> > IST rewrite crud.
> >
>
> This is great, except that the unconditional DR7 write is going to
> seriously hurt perf performance. Fortunately, no one cares about
> perf, right? :)
Good point, so the trivial optimization is below. I couldn't find
instruction latency numbers for DRn load/stores anywhere. I'm hoping
loads are cheap.
> Even just reading first won't help enough because DR7
> reads are likely to be VM exits.
WTF, why is virt always such a horrible piece of crap?
> Can we have a percpu dr7 shadow
> (with careful ordering) or even just a percpu count of dr7 users so we
> can skip this if there are no breakpoints? We have cpu_dr7, and some
> minor changes would make this work. Maybe replace all the direct
> cpu_dr7 access with helpers like dr7_set_bits() and dr7_clear_bits()?
I'll try and sort through that on Monday or so.
---
arch/x86/include/asm/debugreg.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -97,7 +97,8 @@ extern void hw_breakpoint_restore(void);
static __always_inline void local_db_save(unsigned long *dr7)
{
get_debugreg(*dr7, 7);
- set_debugreg(0, 7);
+ if (*dr7)
+ set_debugreg(0, 7);
/*
* Ensure the compiler doesn't lower the above statements into
* the critical section; disabling breakpoints late would not
@@ -114,7 +115,8 @@ static __always_inline void local_db_res
* not be good.
*/
barrier();
- set_debugreg(dr7, 7);
+ if (dr7)
+ set_debugreg(dr7, 7);
}
#ifdef CONFIG_CPU_SUP_AMD
next prev parent reply other threads:[~2020-05-23 13:02 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-05-22 20:47 [RFC][PATCH 0/4] x86/entry: disallow #DB more Peter Zijlstra
2020-05-22 20:47 ` [RFC][PATCH 1/4] x86/entry: Introduce local_db_{rave,restore}() Peter Zijlstra
2020-05-22 20:47 ` [RFC][PATCH 2/4] x86/entry, nmi: Disable #DB Peter Zijlstra
2020-05-22 20:47 ` [RFC][PATCH 3/4] x86/entry: Remove debug IST frobbing Peter Zijlstra
2020-05-22 20:47 ` [RFC][PATCH 4/4] x86/entry, mce: Disallow #DB during #MC Peter Zijlstra
2020-05-22 22:13 ` [RFC][PATCH 0/4] x86/entry: disallow #DB more Andy Lutomirski
2020-05-22 22:20 ` Sean Christopherson
2020-05-22 22:43 ` Andy Lutomirski
2020-05-23 12:59 ` Peter Zijlstra [this message]
2020-05-23 21:32 ` Peter Zijlstra
2020-05-25 10:02 ` Rasmus Villemoes
2020-05-25 10:40 ` Peter Zijlstra
2020-05-25 11:01 ` Peter Zijlstra
2020-05-25 17:19 ` Andy Lutomirski
2020-05-25 18:08 ` Peter Zijlstra
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