Re: [kvm-devel] [PATCH 2/3] Refactor hypercall infrastructure

[Anthony Liguori]

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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