From: Andy Lutomirski <luto@amacapital.net>
To: Sean Christopherson <sean.j.christopherson@intel.com>
Cc: Andy Lutomirski <luto@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
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 15:43:35 -0700 [thread overview]
Message-ID: <692612FC-4599-4774-857C-7E20CD99A832@amacapital.net> (raw)
In-Reply-To: <20200522222055.GE25128@linux.intel.com>
> On May 22, 2020, at 3:20 PM, Sean Christopherson <sean.j.christopherson@intel.com> wrote:
>
> 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? :) 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?
>
> Hmm, I believe hw_breakpoint_active() is what you're looking for, KVM uses
> it to avoid unnecessary restoration of host DR7 after VM-Exit.
>
> Amusingly, checking that in the NMI handler could give a false positive if
> an NMI occurs in guest as DR7 is cleared on exit and KVM invokes the NMI
> handler prior to restoring host DR7. I doubt that's common enough to care
> about though.
False positives are unavoidable: there’s no way we can set a percpu variable and set DR7 without risking an NMI in between.
next prev parent reply other threads:[~2020-05-22 22:43 UTC|newest]
Thread overview: 17+ 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
2020-05-25 11:33 ` kbuild test robot
2020-05-25 12:33 ` kbuild test robot
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 [this message]
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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