On 13/02/13 16:07, Christoffer Dall wrote:
> On Wed, Feb 13, 2013 at 10:46 AM, Marc Zyngier <marc.zyngier@xxxxxxx> wrote:
>> At the moment, KVM/ARM is quite heavy handed when it comes to i-cache
>> invalidation, as it is flushed on each stage-2 mapping.
>>
>> An alternative is to mark each page as non-executable (courtesy of the
>> XN flag), and then to invalidate the i-cache when the CPU tries to
>> execute a page.
>>
>> We're basically trading off invalidation for faults. Performance wise,
>> the performance difference is very modest (I've seen a 0.2%
>> improvement over 10 runs of "hackbench 100 process 1000"). But the
>> important thing in my opinion is that it reduces the impact of the VM
>> on the whole system (fault handling only impact the VM while
>> invalidation is global).
>>
>> Code wise, this introduce a bit of restructuring in our stage-2
>> manipulation code, making the code a bit cleaner (IMHO). Note that
>> these patches are against my arm64 branch, and won't apply on anything
>> else.
>>
>> As always, comments welcome.
>>
>
> Hey Marc,
>
> I'll give this a once-over. From my initial glance there are some
> issues with the stage2_get_pte stuff if you think about section
> mappings later on, and while I kind of see the background, I'm not
> sure it's as bad as it's made out to be.
We certainly need to cater for section mapping, and that would probably
deserve a stage2_{get_pmd,set_pmd_at}.
My problem with the single stage2_set_pte function is that it makes it
very hard to perform a read-modify-write operation on an existing PTE.
We don't really need it now, as we only ever change one single bit (RW).
But as soon as you need a second bit (XN, for example), you're screwed.
The pathological example being a page that has both code and data that
needs to be written.
> In any case, we really need to measure the impact of this on both a
> wider range of workloads inside the VM and on the host performance.
Agreed. I plan to do that at one point once I get all my pending patches
properly aligned (a bit busy with BE guests at the moment).
M.
--
Jazz is not dead. It just smells funny...
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