Re: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault

Xiao Guangrong
2012-05-02T05:28:49Z

On 05/01/2012 09:34 AM, Marcelo Tosatti wrote:

>
> It is getting better, but not yet, there are still reads of sptep
> scattered all over (as mentioned before, i think a pattern of read spte
> once, work on top of that, atomically write and then deal with results
> _everywhere_ (where mmu lock is held) is more consistent.
>

But we only need care the path which depends on is_writable_pte(), no?

So, where call is_writable_pte() are spte_has_volatile_bits(),
spte_write_protect() and set_spte().

I have changed these functions:
In spte_has_volatile_bits():
static bool spte_has_volatile_bits(u64 spte)
{
+ /*
+ * Always atomicly update spte if it can be updated
+ * out of mmu-lock.
+ */
+ if (spte_can_lockless_update(spte))
+ return true;
+

In spte_write_protect():

+ spte = mmu_spte_update(sptep, spte);
+
+ if (is_writable_pte(spte))
+ *flush |= true;
+
The 'spte' is from atomically read-write (xchg).

in set_spte():
set_pte:
- mmu_spte_update(sptep, spte);
+ entry = mmu_spte_update(sptep, spte);
/*
* If we overwrite a writable spte with a read-only one we
* should flush remote TLBs. Otherwise rmap_write_protect
The 'entry' is also the latest value.

> /*
> * If we overwrite a writable spte with a read-only one we
> * should flush remote TLBs. Otherwise rmap_write_protect
> * will find a read-only spte, even though the writable spte
> * might be cached on a CPU's TLB.
> */
> if (is_writable_pte(entry) && !is_writable_pte(*sptep))
> kvm_flush_remote_tlbs(vcpu->kvm);
>
> This is inconsistent with the above obviously.
>

'entry' is not a problem since it is from atomically read-write as
mentioned above, i need change this code to:

/*
* Optimization: for pte sync, if spte was writable the hash
* lookup is unnecessary (and expensive). Write protection
* is responsibility of mmu_get_page / kvm_sync_page.
* Same reasoning can be applied to dirty page accounting.
*/
if (!can_unsync && is_writable_pte(entry) /* Use 'entry' instead of '*sptep'. */
goto set_pte
......

if (is_writable_pte(entry) && !is_writable_pte(spte)) /* Use 'spte' instead of '*sptep'. */
kvm_flush_remote_tlbs(vcpu->kvm);