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SubjectRe: [PATCH] RISC-V: Implement ASID allocator
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I would also like to mention that your code assumes 64-bit atomics 
support which we don't have on 32-bit systems. Using 32-bit to track
ASID generations isn't sufficient and will cause overflows. That's why I
have asid_generation_overflow to handle the case (this is super
error-prone as well).

Best,
Gary

On 27/03/2019 11:42, Anup Patel wrote:
> On Wed, Mar 27, 2019 at 4:57 PM Gary Guo <gary@garyguo.net> wrote:
>>
>> Hi Anup,
>>
>> This won't work in an actual hardware with ASID support. There're more
>
> Can you elaborate why?
>
> This implementation is based on Linux ARM64 ASID allocator which is
> tested for large number of CPUs on real HW.
>
>> interactions with TLB flushes that need to be considered. You won't see
>
> Yap, already considered. Please point me to unhandled case.
>
>> this on both QEMU and SiFive board, as QEMU does not have ASID, so it
>> pretends that ASID is supported by just flushing its TLB everytime you
>
> Nope, it does not. It detects whether ASID is supported or not. If supported
> it will also figure-out number of ASID bits supported by HW.
>
> SiFive board does not have ASID bits so this implementation successfully
> detects that number of ASID bits are 0 and fallbacks to original way of
> local TLB flushes.
>
>> change sptbr. I suspect the performance gain you see is just due to
>> saved TLB flush as TLB flush is super expensive in QEMU (all translation
>> block jumps need to be cleared).
>
> Yes, performance gain is due to saved TLB flushes.
>
> On HW which supports ASID bits, we will see more performance
> improvements.
>
>>
>> I have my version here https://github.com/nbdd0121/linux/tree/asid. I
>> haven't done code cleanups yet, but this version has correctness of its
>> ASID code tested on our TLB simulator tool (which unfortunately I can't
>> share right now as it involves with unpublished works).
>
> Except few minor differences. You version of ASID allocator is same as
> mine. In fact there are lot of similar code framgements in your version
> compared to Linux ARM64 as well. I am sure this patch will work for you.
>
>>
>> In fact my submit my previous patch series exactly as the basis of this
>> patch.
>
> This patch is based your patch series so I suggest you take this patch
> and try it on your simulator.
>
> Suggestions and improvements to this patch are welcomed.
>
> I would be happy if you can assist me to try on your HW.
>
> Regards,
> Anup
>
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