linux-kernel - Re: [Xen-devel] [PATCH 0/3] x86/paravirt: Fix baremetal paravirt MSR ops

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Message-ID: <55FAD83A.3010307@oracle.com>
Date:	Thu, 17 Sep 2015 11:11:54 -0400
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	Andrew Cooper <andrew.cooper3@...rix.com>,
	Andy Lutomirski <luto@...nel.org>
CC:	x86@...nel.org, KVM list <kvm@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, xen-devel <Xen-devel@...ts.xen.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Arjan van de Ven <arjan@...ux.intel.com>
Subject: Re: [Xen-devel] [PATCH 0/3] x86/paravirt: Fix baremetal paravirt
 MSR ops

On 09/17/2015 05:10 AM, Andrew Cooper wrote:
> On 17/09/15 00:33, Andy Lutomirski wrote:
>> Setting CONFIG_PARAVIRT=y has an unintended side effect: it silently
>> turns all rdmsr and wrmsr operations into the safe variants without
>> any checks that the operations actually succeed.
>>
>> This is IMO awful: it papers over bugs.  In particular, KVM gueests
>> might be unwittingly depending on this behavior because
>> CONFIG_KVM_GUEST currently depends on CONFIG_PARAVIRT.  I'm not
>> aware of any such problems, but applying this series would be a good
>> way to shake them out.
>>
>> Fix it so that the MSR operations work the same on CONFIG_PARAVIRT=n
>> and CONFIG_PARAVIRT=y as long as Xen isn't being used.  The Xen
>> maintainers are welcome to make a similar change on top of this.
> The Xen side of things need some further modification before this would
> be a safe operation to perform.
>
> On the wrmsr side of things alone, this is the list of things Xen
> currently objects to and injects #GP faults for.
>
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 00000000c0000081 from
> 0xe023e00800000000 to 0x0023001000000000.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 00000000c0000082 from
> 0xffff82d0bffff000 to 0xffffffff81560060.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 00000000c0000083 from
> 0xffff82d0bffff020 to 0xffffffff81558100.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 0000000000000174 from
> 0x000000000000e008 to 0x0000000000000010.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 0000000000000175 from
> 0xffff8300ac0f7fc0 to 0x0000000000000000.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 0000000000000176 from
> 0xffff82d08023fd50 to 0xffffffff815616d0.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 00000000c0000083 from
> 0xffff82d0bffff020 to 0xffffffff81561910.
> (XEN) traps.c:2692:d0v0 Domain attempted WRMSR 00000000c0000084 from
> 0x0000000000074700 to 0x0000000000047700.
>
> However, it would be certainly be worth teaching PVops not to play with
> MSRs it doesn't own.

PVops already knows about those. There is even has a comment about how 
we shouldn't touch those MSRs. And yet three lines later we still write 
them.

-boris

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