Re: [PATCH v1 09/17] xen/riscv: add helper to check APLIC MSI mode

> This helper can be used outside aplic.c to determine whether MSI mode
> is enabled. A follow-up patch uses it to decide whether the guest
> IMSIC state should be saved/restored.


> 
> Signed-off-by: Oleksii Kurochko <[email protected]>
>
> diff --git a/xen/arch/riscv/aplic.c b/xen/arch/riscv/aplic.c
> index 87f2134bc5..1ce844cd21 100644
> --- a/xen/arch/riscv/aplic.c
> +++ b/xen/arch/riscv/aplic.c
> @@ -93,6 +93,11 @@ void aplic_hw_write_reg(unsigned int offset, uint32_t value)
>      spin_unlock_irqrestore(&aplic.lock, flags);
>  }
>  
> +bool has_msi_support(void)

It does a readl() of the physical APLIC on every call. Patch 11 puts it on
the vCPU context switch path (vaplic_state_save/restore), so this becomes an
uncached MMIO read per switch to read a value that aplic_init_hw_interrupts()
sets once and nothing ever changes. Please cache it at init
time.

-- 
Baptiste Le Duc <[email protected]>