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

[Emilio G. Cota]

On Sun, Oct 07, 2018 at 19:09:01 -0700, Richard Henderson wrote:
> 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 <address@hidden>
> > ---
> >  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.

Aah I see. The point is to avoid pushing to the victim cache
an entry that is all invalid, not just partially invalid.

Thanks for the clarification!

                Emilio