Avi Kivity wrote:
> On 06/02/2010 11:53 AM, Lai Jiangshan wrote:
>> Current code uses slot_bitmap to find ptes who map a page
>> from the memory slot, it is not precise: some ptes in the shadow page
>> are not map any page from the memory slot.
>>
>> This patch uses rmap to find the ptes precisely, and remove
>> the unused slot_bitmap.
>>
>>
>
> Patch looks good; a couple of comments:
>
> - We might see a slowdown with !tdp, since we no longer have locality.
> Each page will map to an spte in a different page. However, it's still
> worth it in my opinion.
Yes, this patch hurts the cache since we no longer have locality.
And if most pages of the slot are not mapped(rmap_next(kvm, rmapp, NULL)==NULL),
this patch will worse than old method I think.
This patch do things straightly, precisely.
> - I thought of a different approach to write protection: write protect
> the L4 sptes, on write fault add write permission to the L4 spte and
> write protect the L3 sptes that it points to, etc. This method can use
> the slot bitmap to reduce the number of write faults. However we can
> reintroduce the slot bitmap if/when we use the method, this shouldn't
> block the patch.
It is very a good approach and it is blazing fast.
I have no time to implement it currently,
could you update it into the TODO list?
>
>>
>> +static void rmapp_remove_write_access(struct kvm *kvm, unsigned long
>> *rmapp)
>> +{
>> + u64 *spte = rmap_next(kvm, rmapp, NULL);
>> +
>> + while (spte) {
>> + /* avoid RMW */
>> + if (is_writable_pte(*spte))
>> + *spte &= ~PT_WRITABLE_MASK;
>
> Must use an atomic operation here to avoid losing dirty or accessed bit.
>
Atomic operation is too expensive, I retained the comment "/* avoid RMW */"
and wait someone take a good approach for it.
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