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SubjectRe: [PATCH v4 06/10] KVM: MMU: fast path of handling guest page fault
On 05/03/2012 05:07 AM, Marcelo Tosatti wrote:

>> '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);
> What is of more importance than the ability to verify that this or that
> particular case are ok at the moment is to write code in such a way that
> its easy to verify that it is correct.
> Thus the suggestion above:
> "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."

Marcelo, thanks for your time to patiently review/reply my mail.

I am confused with ' _everywhere_ ', it means all of the path read/update
spte? why not only verify the path which depends on is_writable_pte()?

For the reason of "its easy to verify that it is correct"? But these
paths are safe since it is not care PT_WRITABLE_MASK at all. What these
paths care is the Dirty-bit and Accessed-bit are not lost, that is why
we always treat the spte as "volatile" if it is can be updated out of

For the further development? We can add the delta comment for
is_writable_pte() to warn the developer use it more carefully.

It is also very hard to verify spte everywhere. :(

Actually, the current code to care PT_WRITABLE_MASK is just for
tlb flush, may be we can fold it into mmu_spte_update.
There are tree ways to modify spte, present -> nonpresent, nonpresent -> present,
present -> present.

But we only need care present -> present for lockless.

* return true means we need flush tlbs caused by changing spte from writeable
* to read-only.
bool mmu_update_spte(u64 *sptep, u64 spte)
u64 last_spte, old_spte = *sptep;
bool flush = false;

last_spte = xchg(sptep, spte);

if ((is_writable_pte(last_spte) ||
spte_has_updated_lockless(old_spte, last_spte)) &&
!is_writable_pte(spte) )
flush = true;

.... track Drity/Accessed bit ...

return flush

Furthermore, the style of "if (spte-has-changed) goto beginning" is feasible
in set_spte since this path is a fast path. (i can speed up mmu_need_write_protect)

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