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

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Message-ID: <55FA8381.5010604@citrix.com>
Date:	Thu, 17 Sep 2015 10:10:25 +0100
From:	Andrew Cooper <andrew.cooper3@...rix.com>
To:	Andy Lutomirski <luto@...nel.org>, <x86@...nel.org>
CC:	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 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.

~Andrew
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