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Message-Id: <1188366670.6337.5.camel@squirrel>
Date: Wed, 29 Aug 2007 00:51:10 -0500
From: Anthony Liguori <anthony@...emonkey.ws>
To: Rusty Russell <rusty@...tcorp.com.au>
Cc: kvm-devel@...ts.sourceforge.net, Avi Kivity <avi@...ranet.com>,
linux-kernel@...r.kernel.org
Subject: Re: [kvm-devel] [PATCH 2/3] Refactor hypercall infrastructure
On Wed, 2007-08-29 at 04:12 +1000, Rusty Russell wrote:
> On Mon, 2007-08-27 at 10:16 -0500, Anthony Liguori wrote:
> > This patch refactors the current hypercall infrastructure to better support live
> > migration and SMP. It eliminates the hypercall page by trapping the UD
> > exception that would occur if you used the wrong hypercall instruction for the
> > underlying architecture and replacing it with the right one lazily.
>
> It also reduces the number of hypercall args, which you don't mention
> here.
Oh yes, sorry.
> > + er = emulate_instruction(&svm->vcpu, kvm_run, 0, 0);
> > +
> > + /* we should only succeed here in the case of hypercalls which
> > + cannot generate an MMIO event. MMIO means that the emulator
> > + is mistakenly allowing an instruction that should generate
> > + a UD fault so it's a bug. */
> > + BUG_ON(er == EMULATE_DO_MMIO);
>
> This seems... unwise. Firstly we know our emulator is incomplete.
> Secondly an SMP guest can exploit this to crash the host.
This code is gone in v2.
> (Code is in two places).
>
> > +#define KVM_HYPERCALL ".byte 0x0f,0x01,0xc1"
Good point.
> A nice big comment would be nice here, I think. Note that this is big
> enough for both "int $0x1f" and "sysenter", so I'm happy.
I need to add a comment somewhere mentioning that if you patch with
something less than 3 bytes, then you should pad with nop but the
hypervisor must treat the whole instruction (including the padding) as
atomic (that is, regardless of hypercall size, eip += 3) or you run the
risk of breakage during migration.
Regards,
Anthony Liguori
> Cheers,
> Rusty.
>
>
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