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From: Andy Lutomirski <luto@kernel.org>
To: Peter Zijlstra <peterz@infradead.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: Fri, 22 May 2020 15:13:57 -0700	[thread overview]
Message-ID: <CALCETrV7GYg5V5dgM9BToc6RAqpcjRdoZoeXbnrTKTqjBfft6g@mail.gmail.com> (raw)
In-Reply-To: <20200522204738.645043059@infradead.org>

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? :)  Even just reading first won't help enough because DR7
reads are likely to be VM exits.  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()?

Also, I like raving at DR7 :)

  parent reply	other threads:[~2020-05-22 22:14 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 ` Andy Lutomirski [this message]
2020-05-22 22:20   ` [RFC][PATCH 0/4] x86/entry: disallow #DB more Sean Christopherson
2020-05-22 22:43     ` Andy Lutomirski
2020-05-23 12:59   ` Peter Zijlstra
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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