Marcelo Tosatti wrote:
>> How about just track access bit for speculative path, we set page both accessed and
>> dirty(if it's writable) only if the access bit is set?
>
> A useful thing to do would be to allow read-only mappings, in the fault
> path (Lai sent a few patches in that direction sometime ago but there
> was no follow up).
>
> So in the case of a read-only fault from the guest, you'd inform
> get_user_pages() that read-only access is acceptable (so swapcache pages
> can be mapped, or qemu can mprotect(PROT_READ) guest memory).
>
Yeah, it's a great work, i guess Lai will post the new version soon.
And, even we do this, i think the page dirty track is still needed, right?
Then, how about my new idea to track page dirty for speculative path, just
as below draft patch does:
@@ -687,10 +687,11 @@ static void drop_spte(struct kvm *kvm, u64 *sptep, u64 new_spte)
if (!is_rmap_spte(old_spte))
return;
pfn = spte_to_pfn(old_spte);
- if (old_spte & shadow_accessed_mask)
+ if (old_spte & shadow_accessed_mask) {
kvm_set_pfn_accessed(pfn);
- if (is_writable_pte(old_spte))
- kvm_set_pfn_dirty(pfn);
+ if (is_writable_pte(old_spte))
+ kvm_set_pfn_dirty(pfn);
+ }
rmap_remove(kvm, sptep);
}
@@ -1920,8 +1921,11 @@ static int set_spte(struct kvm_vcpu *vcpu, u64 *sptep,
* demand paging).
*/
spte = shadow_base_present_pte | shadow_dirty_mask;
- if (!speculative)
+ if (!speculative) {
spte |= shadow_accessed_mask;
+ if (is_writable_pte(*sptep))
+ kvm_set_pfn_dirty(pfn);
+ }
if (!dirty)
pte_access &= ~ACC_WRITE_MASK;
if (pte_access & ACC_EXEC_MASK)
It uses access bit to track both page accessed and page dirty, and it's rather cheap...
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