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Using emap for i386/amd64 early during boot

Hi list,

I finished porting Jeremy's patch for PAE to GENERIC. Most of the patch had to be rewritten, as I had to bring it closer to the way port-xen already handles PAE (initial support was made for Xen, thanks to manuel@), without interfering too much with the current code of port-i386. See [1] for further details about that.

The patch introduces i386_cpu_switch_pmap(), used to hide the logic behind pmap switching between PAE and non-PAE cases. This is motivated by the fact that I had to implement a way of switching pmaps without relying on %cr3 reload, while staying relatively easy to call within asm; see bioscall() and kvm86_call() in [1].

i386_cpu_switch_pmap() uses emap to track TLB flushes (either through tlbflush or lcr3). Unfortunately, bioscall() or kvm86_call() could be called early during boot (before uvm_init()), which will fail badly.

As I have yet to understand the inner workings of emap, I'd like to know if it is possible to wrap i386_cpu_switch_pmap() around uvm_emap functions, like this:

u_int gen = uvm_emap_gen_return();

instead of doing it internally to the function. For the !PAE case, this remains the same, but for PAE, the uvm_emap_gen_return() will be called before the loop that modifies the L3 entries, which spans a splvm(), a loop over 4 entries, and a splx().

What are the constraints about uvm_emap_gen_return/uvm_emap_update? Quick grepping reveals example where they are used just before and after tlbflush/lcr3, but this won't be my case here.

I am open to suggestions... I would prefer to avoid predict_false/predit_true usage here to conditionally avoid calling these when they are not initialized.


For convenience, here's i386_cpu_switch_pmap:

 * Switches pmap for the current CPU. Hides the implementation
 * differences between the PAE and non-PAE cases.
i386_cpu_switch_pmap(struct pmap *pmap)
#ifdef PAE
        int i;
        int s = splvm(); /* just to be safe */
        struct cpu_info *ci = curcpu();
#ifdef XEN
        paddr_t l3_pd = xpmap_ptom_masked(ci->ci_l3_pdirpa);
        /* don't update the kernel L3 slot */
        for (i = 0 ; i < PDP_SIZE - 1; i++) {
                xpq_queue_pte_update(l3_pd + i * sizeof(pd_entry_t),
                    xpmap_ptom(pmap->pm_pdirpa[i]) | PG_V);
#else /* XEN */
        pd_entry_t *l3_pd = ci->ci_l3_pdir;
        for (i = 0 ; i < PDP_SIZE; i++) {
                l3_pd[i] = pmap->pm_pdirpa[i] | PG_V;
#endif /* XEN */

        u_int gen = uvm_emap_gen_return();
#else /* PAE */
        u_int gen = uvm_emap_gen_return();
        lcr3(pmap_pdirpa(pmap, 0));
#endif /* PAE */


Jean-Yves Migeon

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