Re: [Qemu-devel] [PATCH v5 1/3] spapr: introduce a fixed IRQ number space

[David Gibson]

On Thu, Jul 26, 2018 at 03:37:21PM +0200, Cédric Le Goater wrote:
> This proposal introduces a new IRQ number space layout using static
> numbers for all devices, depending on a device index, and a bitmap
> allocator for the MSI IRQ numbers which are negotiated by the guest at
> runtime.
> 
> As the VIO device model does not have a device index but a "reg"
> property, we introduce a formula to compute an IRQ number from a "reg"
> value. It should minimize most of the collisions.
> 
> The previous layout is kept in pre-3.1 machines raising the
> 'legacy_irq_allocation' machine class flag.
> 
> Signed-off-by: Cédric Le Goater <address@hidden>

One nit left..

[snip]
> +static inline uint32_t spapr_vio_reg_to_irq(uint32_t reg)
> +{
> +    uint32_t irq;
> +
> +    if (reg >= SPAPR_VIO_REG_BASE) {
> +        /*
> +         * VIO device register values when allocated by QEMU. For
> +         * these, we simply mask the high bits to fit the overall
> +         * range: [0x00 - 0xff].
> +         *
> +         * The nvram VIO device (reg=0x71000000) is a static device of
> +         * the pseries machine and so is always allocated by QEMU. Its
> +         * IRQ number is 0x0.
> +         */
> +        irq = reg & 0xff;
> +
> +    } else if (reg >= 0x30000000) {
> +        /*
> +         * VIO tty devices register values, when allocated by livirt,
> +         * are mapped in range [0xf0 - 0xff], gives us a maximum of 16
> +         * vtys.
> +         */
> +        irq = 0xf0 | ((reg >> 12) & 0xf);
> +
> +    } else {
> +        /*
> +         * Other VIO devices register values, when allocated by
> +         * livirt, are mapped in range [0x00 - 0xef].
> +         */
> +        irq = (reg >> 12) & 0xef;

This mask doesn't do what you intend - it will map 0x10 to 0, for
example.  You could use % 0xf0, but actually you might as well just
use & 0xff.  Yes, it could collide with the vty devices, but either
way you can still have collisions if you try hard enough.  And, either
way, they'll get detected later.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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