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

[Suzuki K Poulose <suzuki.poulose@arm.com>]

Hi Alexandru

On 22/04/2024 12:58, Alexandru Elisei wrote:
> 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?

No. Realm EL1 must be Aarch64.

> 
>> +
>>   #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?

Do you mean

./configure --earlycon=<NS-Alias-normal-UART-Address> for Realms ?

If so, there are multiple issues with that :

1. A payload could be run in a normal VM or a Realm VM. Having the above
restricts using the same payload in different worlds. (e.g., comparison).

2. The IPA width of the Realm and thus the PTE_NS_SHARED is dynamic
and really depends on what the VMM decides to choose the IPA size.
(Could be based on user input).

If any of the above fails, and a wrong earlycon address could result
in "Synchronouse External Abort" into the Realm (if it is in protected
IPA).

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