From: Andy Lutomirski <luto@amacapital.net>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>,
Andy Lutomirski <luto@kernel.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: Mon, 25 May 2020 10:19:08 -0700 [thread overview]
Message-ID: <2E6DBDE0-FEEA-467F-A380-4ED736B6C912@amacapital.net> (raw)
In-Reply-To: <20200525110101.GG325303@hirez.programming.kicks-ass.net>
> On May 25, 2020, at 4:01 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Mon, May 25, 2020 at 12:40:38PM +0200, Peter Zijlstra wrote:
>>> On Mon, May 25, 2020 at 12:02:48PM +0200, Rasmus Villemoes wrote:
>>>
>>> Naive question: did you check disassembly to see whether gcc threw your
>>> native_get_debugreg() away, given that the asm isn't volatile and the
>>> result is not used for anything? Testing here only shows a "mov
>>> %r9,%db7", but the read did seem to get thrown away.
>>
>> Argh.. no I did not. Writing it all in asm gets me:
>>
>> [ 1.627405] XXX: 3900 8304 22632
>>
>> which is a lot worse...
>
> + u64 empty = 0, read = 0, write = 0, cpu = 0, cpu1 = 0;
> + unsigned long dr7;
> +
> + for (i=0; i<100; i++) {
> + u64 s;
> +
> + s = rdtsc();
> + asm volatile ("lfence; lfence;");
> + empty += rdtsc() - s;
> +
> + s = rdtsc();
> + asm volatile ("lfence; mov %%db7, %0; lfence;" : "=r" (dr7));
> + read += rdtsc() - s;
> +
> + s = rdtsc();
> + asm volatile ("lfence; mov %0, %%db7; lfence;" :: "r" (dr7));
> + write += rdtsc() - s;
> +
> + s = rdtsc();
> + asm volatile ("lfence; mov %0, %%db7; lfence;" :: "r" (dr7));
> + write += rdtsc() - s;
> +
> + clflush(this_cpu_ptr(&cpu_dr7));
> +
> + s = rdtsc();
> + asm volatile ("lfence;");
> + dr7 = this_cpu_read(cpu_dr7);
> + asm volatile ("lfence;");
> + cpu += rdtsc() - s;
> +
> + s = rdtsc();
> + asm volatile ("lfence;");
> + dr7 = this_cpu_read(cpu_dr7);
> + asm volatile ("lfence;");
> + cpu1 += rdtsc() - s;
> + }
> +
> + printk("XXX: %ld %ld %ld %ld %ld\n", empty, read, write, cpu, cpu1);
>
> [ 1.628252] XXX: 3820 8224 45516 35560 4800
>
> Which still seems to suggest using DR7 directly is probably a good
> thing. It's slower than a L1 hit, but massively faster than a full miss.
>
How about adding it to cpu_tlbstate? A lot of NMIs are going to read that anyway to check CR3.
And blaming KVM is a bit misplaced. This isn’t KVM’s fault — it’s Intel’s. VT-x has two modes: DR access exits and DR access doesn’t exit. There’s no shadow mode.
next prev parent reply other threads:[~2020-05-25 17:19 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
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 [this message]
2020-05-25 18:08 ` Peter Zijlstra
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