Re: [Qemu-devel] [RFC 2/6] cputlb: do not evict invalid entries to the vtlb

[Richard Henderson <richard.henderson@linaro.org>]

On 10/6/18 2:45 PM, Emilio G. Cota wrote:
> Currently we evict an entry to the victim TLB when it doesn't match
> the current address. But it could be that there's no match because
> the current entry is invalid. Do not evict the entry to the vtlb
> in that case.
> 
> This change will help us keep track of the TLB's use rate.
> 
> Signed-off-by: Emilio G. Cota <cota@braap.org>
> ---
>  include/exec/cpu-all.h | 14 ++++++++++++++
>  accel/tcg/cputlb.c     |  2 +-
>  2 files changed, 15 insertions(+), 1 deletion(-)
> 
> diff --git a/include/exec/cpu-all.h b/include/exec/cpu-all.h
> index 117d2fbbca..d938dedafc 100644
> --- a/include/exec/cpu-all.h
> +++ b/include/exec/cpu-all.h
> @@ -362,6 +362,20 @@ static inline bool tlb_hit(target_ulong tlb_addr, target_ulong addr)
>      return tlb_hit_page(tlb_addr, addr & TARGET_PAGE_MASK);
>  }
>  
> +/**
> + * tlb_is_valid - return true if at least one of the addresses is valid
> + * @te: pointer to CPUTLBEntry
> + *
> + * This is useful when we don't have a particular address to compare against,
> + * and we just want to know whether any entry holds valid data.
> + */
> +static inline bool tlb_is_valid(const CPUTLBEntry *te)
> +{
> +    return !(te->addr_read & TLB_INVALID_MASK) ||
> +           !(te->addr_write & TLB_INVALID_MASK) ||
> +           !(te->addr_code & TLB_INVALID_MASK);
> +}

No, I think you misunderstand.

First, TLB_INVALID_MASK is only ever set for addr_write, in response to
PAGE_WRITE_INV.  Second, an entry that is invalid for write is still valid for
read+exec.  So there is benefit to swapping it out to the victim cache.

This is used by the s390x target to make the "lowpage" read-only, which is a
special architected 512 byte range within pages 0 and 1.  This is done by
forcing writes, but not reads, back through tlb_fill.


r~