Re: [Qemu-devel] [PATCH] hw/arm/virt: Add always-on property to the virt board timer

[Andrew Jones <drjones@redhat.com>]

On Tue, Jan 19, 2016 at 12:49:18PM +0100, Christoffer Dall wrote:
> The virt board has an arch timer, which is always on.  Emit the
> "always-on" property to indicate to Linux that it can switch off the
> periodic timer and reduces the amount of interrupts injected into a
> guest.
> 
> Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
> ---
>  hw/arm/virt.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/hw/arm/virt.c b/hw/arm/virt.c
> index 05f9087..265fe9a 100644
> --- a/hw/arm/virt.c
> +++ b/hw/arm/virt.c
> @@ -291,6 +291,7 @@ static void fdt_add_timer_nodes(const VirtBoardInfo *vbi, int gictype)
>          qemu_fdt_setprop_string(vbi->fdt, "/timer", "compatible",
>                                  "arm,armv7-timer");
>      }
> +    qemu_fdt_setprop(vbi->fdt, "/timer", "always-on", NULL, 0);
>      qemu_fdt_setprop_cells(vbi->fdt, "/timer", "interrupts",
>                         GIC_FDT_IRQ_TYPE_PPI, ARCH_TIMER_S_EL1_IRQ, irqflags,
>                         GIC_FDT_IRQ_TYPE_PPI, ARCH_TIMER_NS_EL1_IRQ, irqflags,
> -- 
> 2.1.2.330.g565301e.dirty
> 
>

Hi Christoffer,

We should also patch the ACPI generation at the same time. I think
something like

 - gtdt->non_secure_el1_flags = ACPI_EDGE_SENSITIVE;
 + gtdt->non_secure_el1_flags = ACPI_EDGE_SENSITIVE | ACPI_GTDT_ALWAYS_ON;

should do it.

Also, having the guest reduce the number of interrupts sounds good. Can
you point me to something to read about how/why a guest may choose to do
that, and what the trade-offs are?

Thanks,
drew