Re: [Qemu-devel] [PATCH v1] s390x/cpumodel: allow to enable "idtes" feature for TCG

[Thomas Huth <thuth@redhat.com>]

On 27.06.2017 18:10, David Hildenbrand wrote:
> STFL bit 4 and 5 are just indications to the guest, which TLB entries an
> IDTE call will clear. These are performance indicators for the guest.
> 
> STFL bit 4:
>     INVALIDATE DAT TABLE ENTRY (IDTE) performs
>     the invalidation-and-clearing operation by
>     selectively clearing TLB segment-table entries
>     when a segment-table entry or entries are
>     invalidated. IDTE also performs the clearing-by-
>     ASCE operation. Unless bit 4 is one, IDTE simply
>     purges all TLBs. Bit 3 is one if bit 4 is one.
> 
> We can simply set STFL bit 4 ("idtes") and still purge the complete TLB.
> Purging more than advertised is never bad. E.g. Linux doesn't even care
> about this bit. We can optimized this later.

Not sure, but why do we need this bit in add_qemu_cpu_model_features()
if Linux does not care about it? We will get it automatically once we
support the z9 in TCG...

 Thomas