Re: [PATCH] KVM: emulator: Handle wraparound in (cs_base + offset) when fetching.

[Avi Kivity <avi@redhat.com>]

On 04/13/2011 06:44 PM, Nelson Elhage wrote:
> Currently, setting a large (i.e. negative) base address for %cs does not work on
> a 64-bit host. The "JOS" teaching operating system, used by MIT and other
> universities, relies on such segments while bootstrapping its way to full
> virtual memory management.
>
> Signed-off-by: Nelson Elhage<nelhage@ksplice.com>
> ---
>   arch/x86/kvm/emulate.c |    5 ++++-
>   1 files changed, 4 insertions(+), 1 deletions(-)
>
> diff --git a/arch/x86/kvm/emulate.c b/arch/x86/kvm/emulate.c
> index 0ad47b8..54e84b2 100644
> --- a/arch/x86/kvm/emulate.c
> +++ b/arch/x86/kvm/emulate.c
> @@ -505,9 +505,12 @@ static int do_fetch_insn_byte(struct x86_emulate_ctxt *ctxt,
>   	int size, cur_size;
>
>   	if (eip == fc->end) {
> +		unsigned long linear = eip + ctxt->cs_base;
> +		if (ctxt->mode != X86EMUL_MODE_PROT64)
> +			linear&= (u32)-1;
>   		cur_size = fc->end - fc->start;
>   		size = min(15UL - cur_size, PAGE_SIZE - offset_in_page(eip));
> -		rc = ops->fetch(ctxt->cs_base + eip, fc->data + cur_size,
> +		rc = ops->fetch(linear, fc->data + cur_size,
>   				size, ctxt->vcpu,&ctxt->exception);
>   		if (rc != X86EMUL_CONTINUE)
>   			return rc;

A better fix would be to call linearize() here, which does the necessary 
truncation as well as segment checks.

However, this patch is a lot more backportable, so I think it should be 
applied, and a conversion to linearize() performed afterwards.

-- 
error compiling committee.c: too many arguments to function