Re: [PATCH v3] target/s390x: support PRNO_TRNG instruction

[Jason A. Donenfeld]

Hi David,

On Wed, Jul 20, 2022 at 08:41:48PM +0200, David Hildenbrand wrote:
> > +        while (len) {
> > +                size_t block = MIN(len, sizeof(tmp));
> > +
> > +                qemu_guest_getrandom_nofail(tmp, block);
> > +                for (size_t i = 0; i < block; ++i) {
> > +                        cpu_stb_data_ra(env, wrap_address(env, *buf_reg), 
> > tmp[i], ra);
> > +                        *buf_reg = deposit64(*buf_reg, 0, reg_len, 
> > *buf_reg + 1);
> > +                        --*len_reg;
> > +                }
> > +                len -= block;
> > +        }
> 
> Yeah, that's better, although kind-off hard to read.
> 
> We could process one guest page at a time, similar to how we handle
> target/s390x/tcg/mem_helper.c:access_memset and friends nowadays.
> 
> But I won't force you to do that ;)
> 
> This here is good enough for now, with room for improvement regarding
> efficiency.
> 
> I did not review the doc in detail once again, maybe I get to that later
> this week.

Alright, so we'll leave it be for now then and stick with this v3.

The do_access_memset trick is clever, but sheesh, seems a bit overkill
for here. On the real hardware, this instruction takes ~190us for every
32 byte chunk, so there's basically no way that we can possibly be worse
than that. :)

Jason