Re: [PATCH] irqchip/gic-v3-its: add allocation max order limitation for lpi_id_bits

From: Marc Zyngier
Date: Sun Aug 26 2018 - 15:01:34 EST


[I'm travelling, so expect some major delays in responding to email]

Hi Jia,

On Sun, 26 Aug 2018 10:00:51 +0100,
Jia He <hejianet@xxxxxxxxx> wrote:
>
> There is a WARN_ON when my QDF2400 server boots up (pagesize is 4k)

[snip]

> In its_alloc_lpi_tables, lpi_id_bits is 24, without this patch,
> its_allocate_prop_table will try to allocate 16M(order 12 if
> pagesize=4k). Thus it causes the WARN_ON.

Gah! QDF and its 24bit INTIDs... Making life hell for everyone ;-)

Sorry for breaking it.

>
> This patch fixes it by limiting the lpi_id_bits.
>
> Signed-off-by: Jia He <jia.he@xxxxxxxxxxxxxxxx>
> ---
> drivers/irqchip/irq-gic-v3-its.c | 5 ++++-
> 1 file changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/irqchip/irq-gic-v3-its.c b/drivers/irqchip/irq-gic-v3-its.c
> index 316a575..79e6993 100644
> --- a/drivers/irqchip/irq-gic-v3-its.c
> +++ b/drivers/irqchip/irq-gic-v3-its.c
> @@ -1624,8 +1624,11 @@ static void its_free_prop_table(struct page *prop_page)
> static int __init its_alloc_lpi_tables(void)
> {
> phys_addr_t paddr;
> + u32 max_bits; /*max order limitation in alloc_page*/
>
> - lpi_id_bits = GICD_TYPER_ID_BITS(gic_rdists->gicd_typer);
> + max_bits = PAGE_SHIFT + MAX_ORDER - 1;
> + lpi_id_bits = min_t(u32, max_bits,
> + GICD_TYPER_ID_BITS(gic_rdists->gicd_typer));
> gic_rdists->prop_page = its_allocate_prop_table(GFP_NOWAIT);
> if (!gic_rdists->prop_page) {
> pr_err("Failed to allocate PROPBASE\n");
> --
> 1.8.3.1
>

I find it rather odd that we end-up with different interrupt ranges
depending on the CPU page size. Also, allocating that much memory for
LPIs is rather pointless, as we actually have a pretty low limit of
interrupts the system can deal with (see IRQ_BITMAP_BITS, which is
slightly more than 8k). I've so far seen *one* request to push it up,
but I doubt that it is a real use case.

Capping lpi_id_bits at 16 (which is what we had before) is plenty,
will save a some memory, and gives some margin before we need to push
it up again.

Thanks,

M.

--
Jazz is not dead, it just smell funny.