Re: [kvm-unit-tests PATCH 08/33] arm: realm: Make uart available before MMU is enabled

[Alexandru Elisei <alexandru.elisei@arm.com>]

Hi,

On Fri, Apr 12, 2024 at 11:33:43AM +0100, Suzuki K Poulose wrote:
> From: Joey Gouly <joey.gouly@arm.com>
> 
> A Realm must access any emulated I/O mappings with the PTE_NS_SHARED bit set.
> This is modelled as a PTE attribute, but is actually part of the address.
> 
> So, when MMU is disabled, the "physical address" must reflect this bit set. We
> access the UART early before the MMU is enabled. So, make sure the UART is
> accessed always with the bit set.
> 
> Signed-off-by: Joey Gouly <joey.gouly@arm.com>
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> ---
>  lib/arm/asm/pgtable.h   |  5 +++++
>  lib/arm/io.c            | 24 +++++++++++++++++++++++-
>  lib/arm64/asm/pgtable.h |  5 +++++
>  3 files changed, 33 insertions(+), 1 deletion(-)
> 
> diff --git a/lib/arm/asm/pgtable.h b/lib/arm/asm/pgtable.h
> index 350039ff..7e85e7c6 100644
> --- a/lib/arm/asm/pgtable.h
> +++ b/lib/arm/asm/pgtable.h
> @@ -112,4 +112,9 @@ static inline pte_t *pte_alloc(pmd_t *pmd, unsigned long addr)
>  	return pte_offset(pmd, addr);
>  }
>  
> +static inline unsigned long arm_shared_phys_alias(void *x)
> +{
> +	return ((unsigned long)(x) | PTE_NS_SHARED);
> +}

Is it allowed for a realm to run in aarch32 mode?

> +
>  #endif /* _ASMARM_PGTABLE_H_ */
> diff --git a/lib/arm/io.c b/lib/arm/io.c
> index 836fa854..127727e4 100644
> --- a/lib/arm/io.c
> +++ b/lib/arm/io.c
> @@ -15,6 +15,8 @@
>  #include <asm/psci.h>
>  #include <asm/spinlock.h>
>  #include <asm/io.h>
> +#include <asm/mmu-api.h>
> +#include <asm/pgtable.h>
>  
>  #include "io.h"
>  
> @@ -30,6 +32,24 @@ static struct spinlock uart_lock;
>  static volatile u8 *uart0_base = UART_EARLY_BASE;
>  bool is_pl011_uart;
>  
> +static inline volatile u8 *get_uart_base(void)
> +{
> +	/*
> +	 * The address of the UART base may be different
> +	 * based on whether we are running with/without
> +	 * MMU enabled.
> +	 *
> +	 * For realms, we must force to use the shared physical
> +	 * alias with MMU disabled, to make sure the I/O can
> +	 * be emulated.
> +	 * When the MMU is turned ON, the mappings are created
> +	 * appropriately.
> +	 */
> +	if (mmu_enabled())
> +		return uart0_base;
> +	return (u8 *)arm_shared_phys_alias((void *)uart0_base);
> +}
> +
>  static void uart0_init_fdt(void)
>  {
>  	/*
> @@ -109,9 +129,11 @@ void io_init(void)
>  
>  void puts(const char *s)
>  {
> +	volatile u8 *uart_base = get_uart_base();
> +
>  	spin_lock(&uart_lock);
>  	while (*s)
> -		writeb(*s++, uart0_base);
> +		writeb(*s++, uart_base);
>  	spin_unlock(&uart_lock);
>  }
>  
> diff --git a/lib/arm64/asm/pgtable.h b/lib/arm64/asm/pgtable.h
> index 5b9f40b0..871c03e9 100644
> --- a/lib/arm64/asm/pgtable.h
> +++ b/lib/arm64/asm/pgtable.h
> @@ -28,6 +28,11 @@ extern unsigned long prot_ns_shared;
>  */
>  #define PTE_NS_SHARED		(prot_ns_shared)
>  
> +static inline unsigned long arm_shared_phys_alias(void *addr)
> +{
> +	return ((unsigned long)addr | PTE_NS_SHARED);
> +}

Have you considered specifying the correct UART address at compile time using
./configure --earlycon?

Thanks,
Alex
> +
>  /*
>   * Highest possible physical address supported.
>   */
> -- 
> 2.34.1
>