Re: [PATCH 2/3] Refactor hypercall infrastructure

[Rusty Russell <rusty@rustcorp.com.au>]

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.
	
> +	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.

(Code is in two places).

> +#define KVM_HYPERCALL ".byte 0x0f,0x01,0xc1"

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.

Cheers,
Rusty.