Re: [Xen-devel] [PATCH] Xen: enhanced PV on HVM

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 16:05 +0000 on 09 Mar (1268150723), Stefano Stabellini wrote:
> @@ -686,7 +696,16 @@
> 
>      if ( is_hvm_vcpu(v) )
>      {
> +        unsigned long eip, cs;
>          hvm_set_info_guest(v);
> +
> +        eip = c(user_regs.eip);
> +        if (eip != 0) {
> +            cs = eip >> 12 << 8;
> +            hvm_vcpu_reset_state(v, cs, 0);
> +            hvm_funcs.set_tsc_offset(v, 0);

If there isn't a more sensible thing to gate this on, it needs a big
comment explaining why (%eip != 0) ==> (tsc_offset == 0).

Likewise the implicit setting of tsc_offset to zero when setting up the
timer virq needs a comment, and preferably this should all be described
in the public headers too.

The potential in this patch for a guest to get surprised when one of its
vcpus' TSC offset changes under its feet is a bit worrying.

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, XenServer Engineering
Citrix Systems UK Ltd.  (Company #02937203, SL9 0BG)

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