On Sat, Apr 21, 2012 at 11:24:54AM +0800, Xiao Guangrong wrote:
> On 04/21/2012 05:33 AM, Marcelo Tosatti wrote:
>
>
> >> static bool
> >> __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
> >> {
> >> @@ -1050,24 +1078,13 @@ __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp, int level)
> >>
> >> for (sptep = rmap_get_first(*rmapp, &iter); sptep;) {
> >> BUG_ON(!(*sptep & PT_PRESENT_MASK));
> >> - rmap_printk("rmap_write_protect: spte %p %llx\n", sptep, *sptep);
> >> -
> >> - if (!is_writable_pte(*sptep)) {
> >> - sptep = rmap_get_next(&iter);
> >> - continue;
> >> - }
> >> -
> >> - if (level == PT_PAGE_TABLE_LEVEL) {
> >> - mmu_spte_update(sptep, *sptep & ~PT_WRITABLE_MASK);
> >> - sptep = rmap_get_next(&iter);
> >> - } else {
> >> - BUG_ON(!is_large_pte(*sptep));
> >> - drop_spte(kvm, sptep);
> >> - --kvm->stat.lpages;
> >
> > It is preferable to remove all large sptes including read-only ones, the
>
>
> It can cause page faults even if read memory on these large sptse.
>
> Actually, Avi suggested that make large writable spte to be readonly
> (not dropped) on this path.
See commits e49146dce8c3dc6f4485c1904b6587855f393e71,
38187c830cab84daecb41169948467f1f19317e3 for issues
with large read-only sptes.
> > current behaviour, then to verify that no read->write transition can
> > occur in fault paths (fault paths which are increasing in number).
>
>
> Yes, the small spte also has issue (find a write-protected spte in
> fault paths). Later, the second part of this patchset will introduce
> rmap.WRITE_PROTECTED bit, then we can do the fast check before calling
> fast page fault.
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