Re: [PATCH 1/1] irqchip/loongson-pch-pic: Fix vec_count reading for 32-bit and 64-bit

[Huacai Chen]

On Fri, Apr 10, 2026 at 11:02 PM Thomas Gleixner <tglx@xxxxxxxxxx> wrote:
>
> On Fri, Apr 10 2026 at 09:30, George Guo wrote:
> > From: George Guo <guodongtai@xxxxxxxxxx>
> >
> > Commit 0370a5e740f2 ("irqchip/loongson-pch-pic: Adjust irqchip driver for
> > 32BIT/64BIT") changed vec_count reading from readq() to readl() to support
> > both 32-bit and 64-bit platforms. However, on virtual 64-bit platforms
> > (QEMU 8.2.0) this causes incorrect vec_count value, leading to panic:
>
> Is this problem limited to qemu?
I think it is a qemu bug rather than a kernel bug. Since qemu 8.2.0 is
old, I suggest use qemu 10.2.0 to test.


Huacai

>
> > WARNING: drivers/acpi/irq.c:63 at acpi_register_gsi+0xe8/0x108
> > Call Trace:
> > [<900000000024c634>] show_stack+0x64/0x188
> > [<9000000000245154>] dump_stack_lvl+0x6c/0x9c
>
> Please trim your backtrace as documented:
>
> https://www.kernel.org/doc/html/latest/process/submitting-patches.html#backtraces
>
> > @@ -343,7 +343,12 @@ static int pch_pic_init(phys_addr_t addr, unsigned long size, int vec_base,
> >               priv->table[i] = PIC_UNDEF_VECTOR;
> >
> >       priv->ht_vec_base = vec_base;
> > -     priv->vec_count = ((readl(priv->base + 4) >> 16) & 0xff) + 1;
> > +
> > +     if (IS_ENABLED(CONFIG_64BIT))
> > +             priv->vec_count = ((readq(priv->base) >> 48) & 0xff) + 1;
> > +     else
> > +             priv->vec_count = ((readl(priv->base + 4) >> 16) & 0xff) + 1;
>
> This does not make sense at all.
>
>      readl(base + 4) >> 16
>
> is fully equivalent to
>
>      readq(base) >> 48
>
> on a little endian machine, no?
>
> This needs a better explanation in the change log about the root cause
> and why this is the correct solution to fix the problem.
>
> If there is no other solution then this needs a big fat comment in the
> code explaining the reason. Otherwise the next AI agent will notice the
> equivalence and people will send cleanup patches....
>
> Thanks,
>
>         tglx