On 24.07.2013, at 12:01, Gleb Natapov wrote:
> Copying Andrea for him to verify that I am not talking nonsense :)
>
> On Wed, Jul 24, 2013 at 10:25:20AM +0200, Alexander Graf wrote:
>>> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
>>> index 1580dd4..5e8635b 100644
>>> --- a/virt/kvm/kvm_main.c
>>> +++ b/virt/kvm/kvm_main.c
>>> @@ -102,6 +102,10 @@ static bool largepages_enabled = true;
>>>
>>> bool kvm_is_mmio_pfn(pfn_t pfn)
>>> {
>>> +#ifdef CONFIG_MEMORY_HOTPLUG
>>
>> I'd feel safer if we narrow this down to e500.
>>
>>> + /*
>>> + * Currently only in memory hot remove case we may still need this.
>>> + */
>>> if (pfn_valid(pfn)) {
>>
>> We still have to check for pfn_valid, no? So the #ifdef should be down here.
>>
>>> int reserved;
>>> struct page *tail = pfn_to_page(pfn);
>>> @@ -124,6 +128,7 @@ bool kvm_is_mmio_pfn(pfn_t pfn)
>>> }
>>> return PageReserved(tail);
>>> }
>>> +#endif
>>>
>>> return true;
>>> }
>>>
>>> Before apply this change:
>>>
>>> real (1m19.954s + 1m20.918s + 1m22.740s + 1m21.146s + 1m22.120s)/5= 1m21.376s
>>> user (0m23.181s + 0m23.550s + 0m23.506s + 0m23.410s + 0m23.520s)/5= 0m23.433s
>>> sys (0m49.087s + 0m49.563s + 0m51.758s + 0m50.290s + 0m51.047s)/5= 0m50.349s
>>>
>>> After apply this change:
>>>
>>> real (1m19.507s + 1m20.919s + 1m21.436s + 1m21.179s + 1m20.293s)/5= 1m20.667s
>>> user (0m22.595s + 0m22.719s + 0m22.484s + 0m22.811s + 0m22.467s)/5= 0m22.615s
>>> sys (0m48.841s + 0m49.929s + 0m50.310s + 0m49.813s + 0m48.587s)/5= 0m49.496s
>>>
>>> So,
>>>
>>> real (1m20.667s - 1m21.376s)/1m21.376s x 100% = -0.6%
>>> user (0m22.615s - 0m23.433s)/0m23.433s x 100% = -3.5%
>>> sys (0m49.496s - 0m50.349s)/0m50.349s x 100% = -1.7%
>>
>> Very nice, so there is a real world performance benefit to doing this. Then yes, I think it would make sense to change the global helper function to be fast on e500 and use that one from e500_shadow_mas2_attrib() instead.
>>
>> Gleb, Paolo, any hard feelings?
>>
> I do not see how can we break the function in such a way and get
> away with it. Not all valid pfns point to memory. Physical address can
> be sparse (due to PCI hole, framebuffer or just because).
But we don't check for sparseness today in here either. We merely check for incomplete huge pages.
Alex
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