Re: [PATCH v7 2/2] target/s390x: support PRNO_TRNG instruction

[Thomas Huth]

On 29/08/2022 18.29, Jason A. Donenfeld wrote:
> On Fri, Aug 26, 2022 at 01:28:11PM +0200, Thomas Huth wrote:
> > > +        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, message_reg_len, *buf_reg + 1);
> > > +            --*len_reg;
> >
> > I know it's annoying, but technically, you must not touch the upper bits of
> > the len_reg if running in 31- or 24-bit addressing mode. The Principles of
> > Operations say:
> >
> > "In either the 24- or 31-bit addressing mode, bits 32-63 of the odd-numbered
> > register are decremented by the number
> > of bytes processed for the respective operand, and
> > bits 0-31 of the register remain unchanged."
>
> This is what I was trying to do with the use of deposit64, following
> David's guidance. Did I mess something up?

Sorry for not following up earlier - I've been away from keyboard for a 
couple of weeks...

Anyway, that was likely a wrong comment from my side anyway - I thought that 
"--*len_reg" might alter the upper bits, too, when there is no masking here. 
But since "len" has been constrained earlier in the function already, I 
think this cannot happen, so please never mind.

I just saw that you also sent a v8 now, so I'll follow up on that version.

 Thomas