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From: Linus Torvalds <torvalds@linux-foundation.org>
To: Andy Lutomirski <luto@amacapital.net>
Cc: KVM list <kvm@vger.kernel.org>,
	Peter Zijlstra <peterz@infradead.org>, X86 ML <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	xen-devel <Xen-devel@lists.xen.org>,
	Borislav Petkov <bp@alien8.de>, Andy Lutomirski <luto@kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arjan van de Ven <arjan@linux.intel.com>
Subject: Re: [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops
Date: Mon, 14 Mar 2016 11:10:59 -0700	[thread overview]
Message-ID: <CA+55aFxL5AT-SM-pQLqbObaCwL=X4Q04snFkg882wVMB=3pWQA__24671.1676977158$1457979147$gmane$org@mail.gmail.com> (raw)
In-Reply-To: <CA+55aFwr80aZ3-1H4VPB5=jeU_Yt=hYFfFvZC5-cNXzgXxGf3A@mail.gmail.com>

On Mon, Mar 14, 2016 at 11:04 AM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> None of this insane complicated crap that buys us exactly *nothing*,
> and depends on fancy new exception handling support etc etc.

Actually, the one _new_ thing we could do is to instead of removing
the old crappy rdmsr()/wrmsr() implementation entirely, we'll just
rename it to "rd/wrmsr_unsafe()", and not have any exception table
stuff for that at all (so now you *will* get an oops and panic if you
use that).

The only reason to do that is for performance-critical MSR's that
really don't want any overhead. Sure, they could just be changed to
use "wrmsr_safe()", but for things like the APIC accesses, or
update_debugctlmsr() (that is supposed to check for processor version)
that can be truly critical, an explicitly _unsafe_ version may be the
right thing to do.

The fact is, the problem with rd/wrmsr is that we just did the
defaults the wrong way around. Making the simple and obvious version
be unsafe is just wrong.


Xen-devel mailing list

  parent reply	other threads:[~2016-03-14 18:10 UTC|newest]

Thread overview: 23+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <cover.1457805972.git.luto@kernel.org>
2016-03-12 18:08 ` [PATCH v4 1/5] x86/paravirt: Add _safe to the read_msr and write_msr PV hooks Andy Lutomirski
2016-03-12 18:08 ` [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops Andy Lutomirski
2016-03-12 18:08 ` [PATCH v4 3/5] x86/paravirt: Add paravirt_{read, write}_msr Andy Lutomirski
2016-03-12 18:08 ` [PATCH v4 4/5] x86/paravirt: Make "unsafe" MSR accesses unsafe even if PARAVIRT=y Andy Lutomirski
2016-03-12 18:08 ` [PATCH v4 5/5] x86/msr: Set the return value to zero when native_rdmsr_safe fails Andy Lutomirski
     [not found] ` <7e72cd6ce08b946872a462fad13eca1810b8671d.1457805972.git.luto@kernel.org>
2016-03-14 11:57   ` [PATCH v4 1/5] x86/paravirt: Add _safe to the read_msr and write_msr PV hooks Borislav Petkov
     [not found]   ` <20160314115729.GC15800@pd.tnic>
2016-03-14 17:07     ` Andy Lutomirski
     [not found] ` <a3b871a4eb533340d04255409dfecc94f88c647d.1457805972.git.luto@kernel.org>
2016-03-14 12:02   ` [PATCH v4 2/5] x86/msr: Carry on after a non-"safe" MSR access fails without !panic_on_oops Borislav Petkov
     [not found]   ` <20160314120202.GD15800@pd.tnic>
2016-03-14 17:05     ` Andy Lutomirski
     [not found]     ` <CALCETrW6E0Nz6gSmRKTvHbQDhnHVpuhzmgZB1nZ3m-DL-Bt=tQ@mail.gmail.com>
2016-03-14 17:11       ` Linus Torvalds
2016-03-14 17:17         ` Andy Lutomirski
     [not found]         ` <CALCETrXhXPj_b6rUMn=SR0QwE92rL=k5DCFraZwBj9FpUgadYw@mail.gmail.com>
2016-03-14 18:04           ` Linus Torvalds
     [not found]           ` <CA+55aFwr80aZ3-1H4VPB5=jeU_Yt=hYFfFvZC5-cNXzgXxGf3A@mail.gmail.com>
2016-03-14 18:10             ` Andy Lutomirski
2016-03-14 18:10             ` Linus Torvalds [this message]
     [not found]             ` <CALCETrV16vh5U0TRzdroSTBR_QHDX1C78t0DDW9qtKjOmV+2sQ@mail.gmail.com>
2016-03-14 18:15               ` Linus Torvalds
     [not found]               ` <CA+55aFz=PdvMWSsXExrpQC2UV6aLS+=+VOo+K4=njQoU5B9hgA@mail.gmail.com>
2016-03-14 18:24                 ` Andy Lutomirski
     [not found]                 ` <CALCETrXAAOcGP7DK+7aKn=2pu=SQ0n_PhG9bV4DcoYcv9epn4A@mail.gmail.com>
2016-03-14 18:40                   ` Linus Torvalds
     [not found]                   ` <CA+55aFy2dNgBJwjd+fRUYQf9OxqGkkcjUEarv+o3QSFTJ6+gJQ@mail.gmail.com>
2016-03-14 18:48                     ` Andy Lutomirski
     [not found]                     ` <CALCETrWxSBwwFTFsRoR_9AVBoC0iJ1pdWwOq_cnFf3NHUpG4QA@mail.gmail.com>
2016-03-15 10:22                       ` Ingo Molnar
     [not found]                       ` <20160315102230.GB23406@gmail.com>
2016-03-15 10:26                         ` Ingo Molnar
2016-03-14 20:18               ` Peter Zijlstra
2016-03-15 10:21             ` Ingo Molnar
2016-03-14 14:32 ` [PATCH v4 0/5] [PATCH v3 0/5] Improve non-"safe" MSR access failure handling Boris Ostrovsky

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