Re: [PATCH v6 23/23] hw/riscv: virt: Increase maximum number of allowed CPUs

From: Alistair Francis <alistair23@gmail.com>
To: Anup Patel <anup@brainfault.org>
Cc: Peter Maydell <peter.maydell@linaro.org>,
	"open list:RISC-V" <qemu-riscv@nongnu.org>,
	Sagar Karandikar <sagark@eecs.berkeley.edu>,
	"qemu-devel@nongnu.org Developers" <qemu-devel@nongnu.org>,
	Alistair Francis <Alistair.Francis@wdc.com>,
	Atish Patra <atishp@atishpatra.org>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Bin Meng <bmeng.cn@gmail.com>
Subject: Re: [PATCH v6 23/23] hw/riscv: virt: Increase maximum number of allowed CPUs
Date: Thu, 6 Jan 2022 07:50:20 +1000	[thread overview]
Message-ID: <CAKmqyKNWfKb26BeZ1LSE6OqmcbgnoUpAqX2XhP4i9-kCf6NCFw@mail.gmail.com> (raw)
In-Reply-To: <20211230123539.52786-24-anup@brainfault.org>

On Thu, Dec 30, 2021 at 11:06 PM Anup Patel <anup@brainfault.org> wrote:
>
> From: Anup Patel <anup.patel@wdc.com>
>
> To facilitate software development of RISC-V systems with large number
> of HARTs, we increase the maximum number of allowed CPUs to 512 (2^9).
>
> We also add a detailed source level comments about limit defines which
> impact the physical address space utilization.
>
> Signed-off-by: Anup Patel <anup.patel@wdc.com>
> Signed-off-by: Anup Patel <anup@brainfault.org>

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>

Alistair

> ---
>  hw/riscv/virt.c         | 10 ++++++++++
>  include/hw/riscv/virt.h |  2 +-
>  2 files changed, 11 insertions(+), 1 deletion(-)
>
> diff --git a/hw/riscv/virt.c b/hw/riscv/virt.c
> index 7579f7b41f..86b03736eb 100644
> --- a/hw/riscv/virt.c
> +++ b/hw/riscv/virt.c
> @@ -44,6 +44,16 @@
>  #include "hw/pci-host/gpex.h"
>  #include "hw/display/ramfb.h"
>
> +/*
> + * The virt machine physical address space used by some of the devices
> + * namely ACLINT, PLIC, APLIC, and IMSIC depend on number of Sockets,
> + * number of CPUs, and number of IMSIC guest files.
> + *
> + * Various limits defined by VIRT_SOCKETS_MAX_BITS, VIRT_CPUS_MAX_BITS,
> + * and VIRT_IRQCHIP_MAX_GUESTS_BITS are tuned for maximum utilization
> + * of virt machine physical address space.
> + */
> +
>  #define VIRT_IMSIC_GROUP_MAX_SIZE      (1U << IMSIC_MMIO_GROUP_MIN_SHIFT)
>  #if VIRT_IMSIC_GROUP_MAX_SIZE < \
>      IMSIC_GROUP_SIZE(VIRT_CPUS_MAX_BITS, VIRT_IRQCHIP_MAX_GUESTS_BITS)
> diff --git a/include/hw/riscv/virt.h b/include/hw/riscv/virt.h
> index e12e8ddcae..62d8e9c6d0 100644
> --- a/include/hw/riscv/virt.h
> +++ b/include/hw/riscv/virt.h
> @@ -24,7 +24,7 @@
>  #include "hw/block/flash.h"
>  #include "qom/object.h"
>
> -#define VIRT_CPUS_MAX_BITS             3
> +#define VIRT_CPUS_MAX_BITS             9
>  #define VIRT_CPUS_MAX                  (1 << VIRT_CPUS_MAX_BITS)
>  #define VIRT_SOCKETS_MAX_BITS          2
>  #define VIRT_SOCKETS_MAX               (1 << VIRT_SOCKETS_MAX_BITS)
> --
> 2.25.1
>
>