warning of pending commit attempt.

Marko Zec zec at icir.org
Thu Feb 28 14:31:38 UTC 2008

On Thursday 28 February 2008 14:17:04 Dag-Erling Smørgrav wrote:
> Marko Zec <zec at icir.org> writes:
> > [about vimage]
> One thing you haven't mentioned is sysctl.  I've always been of the
> opinion that if we virtualize one part of the system, we should also
> virtualize the sysctl tree.  This does not mean that each vimage
> should have its own copy of the entire tree, but rather that it
> should be possible to mark some sysctl nodes as virtualized.  For
> instance, it would be useful on amd64 to be able to create an i386
> vimage, where hw.machine and hw.machine_arch would be "i386".
> For PROC nodes, of course, this is easily done (as you already do)
> with INIT_VPROCG(TD_TO_VPROGC(curthread)), but the basic node types
> (int, long, string etc.) are a little trickier, and don't seem to be
> handled in your patch.

Actually the patch provides certain level of support for virtualizing 
leaf sysctl nodes.  So far I have only introduced macros for methods / 
data types that I've found necessary to virtualize, such as 
Instead of embedding a "pointer" to the variable that a sysctl needs to 
control into the linker set that standard SYSCTL macros create, the 
SYSCTL_V macros embed an ID of the virtualization container / strucrure 
where the data has to be resolved, and an offset inside such a 

Here's a randomly picked example from a recent diff:

+#ifndef VIMAGE
 struct	icmpstat icmpstat;
-	&icmpstat, icmpstat, "");
+SYSCTL_V_STRUCT(V_NET, vnet_inet, _net_inet_icmp, ICMPCTL_STATS, stats,
+		CTLFLAG_RW, icmpstat, icmpstat, "");

So the latter macro will embedd an offsetof(struct vnet_inet, _icmpstat) 
somewhere in the resulting linker set structure, with V_NET telling us 
that we are resolving a networking symbol, and vnet_inet will also be 
translated to an ID determining in which networking submodule to 
resolve the address.

The above sysctl will be handled by the sysctl_handle_v_int() function:

#ifdef VIMAGE
        int tmpout, error = 0;


         * Attempt to get a coherent snapshot by making a copy of the 
        tmpout = *(int *)arg1;
        error = SYSCTL_OUT(req, &tmpout, sizeof(int));

        if (error || !req->newptr)
                return (error);

        if (!arg1)
                error = EPERM;
                error = SYSCTL_IN(req, arg1, sizeof(int));
        return (error);

Observe the SYSCTL_RESOLVE_V_ARG1() macro above which does the actual 
magic of computing the address of the target variable:

 * Resolve void *arg1 in a proper virtualization container.
#ifdef VIMAGE
#define SYSCTL_RESOLVE_V_ARG1() do {
        char *cp;
        switch (subs) {
        case V_NET:
		cp = (char *) TD_TO_VNET(curthread)->mod_data[mod];
        case V_PROCG:
                cp = (char *) TD_TO_VPROCG(curthread);
        case V_CPU:
                cp = (char *) TD_TO_VCPU(curthread);
                panic("unsupported module id %d", subs);
        arg1 = cp + (size_t) arg1;
} while (0)

Hmm probably I should make subs and mod the arguments of the above 

Anyhow if the kernel is compiled without "options VIMAGE" the SYSCTL_V 
macros fall back to standard SYSCTL macros of course.

Hope this clarifies the picture to some extent...


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