Re: [PATCH] hw/intc/arm_gicv5: Make IRS_IDR0 reads return actual value, not 0

[Jim MacArthur <jim.macarthur@linaro.org>]

On Thu, Oct 08, 2026 at 11:36:59AM +0200, Philippe Mathieu-Daudé wrote:
> On 8/10/26 11:23, Peter Maydell wrote:
> > IN the code for handling reads of IRS_IDR0, we carefully construct
> > the correct value starting with cs->irs_idr0 and adjusting it.
> > However, we never actually write the final value into *data, so
> > the guest will always see zero.
> > 
> > We didn't notice this before, because Linux happens to not care
> > about this register value except to check for the presence of
> > virtualization.
> > 
> > Cc: qemu-stable@nongnu.org
> > Fixes: 405ad2e76f13254 ("hw/intc/arm_gicv5: Implement IRS ID regs")
> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > ---
> >   hw/intc/arm_gicv5.c | 1 +
> >   1 file changed, 1 insertion(+)
> > 
> > diff --git a/hw/intc/arm_gicv5.c b/hw/intc/arm_gicv5.c
> > index 08878193dfc..ebfb0878af3 100644
> > --- a/hw/intc/arm_gicv5.c
> > +++ b/hw/intc/arm_gicv5.c
> > @@ -1433,6 +1433,7 @@ static bool config_readl(GICv5 *s, GICv5Domain domain, hwaddr offset,
> >               /* ...which means VIRT_ONE_N is also RES0 */
> >               v &= ~R_IRS_IDR0_VIRT_ONE_N_MASK;
> >           }
> 
> Looking at the code path it is obvious, I wonder why compilers or
> coverity didn't warned 'v' is set but not used...

AFAICT, 'param @= RHS' counts as a use with -Wunused-but-set-variable in GCC up to version 15; GCC 16.1 adds an exception for this and should warn.

Jim