Re: [QUESTION FOR ARM64 TLB] performance issue and implementation difference of TLB flush

[Gang Li]

Hi all!

On 2023/5/5 20:28, Gang Li wrote:
> Hi,
>
> I found that in `ghes_unmap` protected by spinlock, arm64 and x86 have
> different strategies for flushing tlb.
>
> # arm64 call trace:
> ```
> holding a spin lock
> ghes_unmap
>   clear_fixmap
>    __set_fixmap
>     flush_tlb_kernel_range
> ```
>
> # x86 call trace:
> ```
> holding a spin lock
> ghes_unmap
>   clear_fixmap
>    __set_fixmap
>     mmu.set_fixmap
>      native_set_fixmap
>       __native_set_fixmap
>        set_pte_vaddr
>         set_pte_vaddr_p4d
>          __set_pte_vaddr
>           flush_tlb_one_kernel
> ```
>
> arm64 broadcast TLB invalidation in ghes_unmap, because TLB entry can be
> allocated regardless of whether the CPU explicitly accesses memory.
>
> Why doesn't x86 broadcast TLB invalidation in ghes_unmap? Is there any
> difference between x86 and arm64 in TLB allocation and invalidation 
> strategy?

I found this in Intel® 64 and IA-32 Architectures Software Developer
Manuals:

> 4.10.2.3 Details of TLB Use
> Subject to the limitations given in the previous paragraph, the
> processor may cache a translation for any linear address, even if that
> address is not used to access memory. For example, the processor may
> cache translations required for prefetches and for accesses that result
> from speculative execution that would never actually occur in the
> executed code path.

Both x86 and arm64 can cache TLB for prefetches and speculative
execution. Then why are their flush policies different?

Thanks,
Gang Li