Re: [PATCH kvm-unit-tests 8/8] arm/arm64: psci: don't assume method is hvc

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

Hi Drew,

On 4/7/21 7:59 PM, Andrew Jones wrote:
> The method can also be smc and it will be when running on bare metal.
>
> Signed-off-by: Andrew Jones <drjones@redhat.com>
> ---
>  arm/selftest.c     | 34 +++++++---------------------------
>  lib/arm/asm/psci.h |  9 +++++++--
>  lib/arm/psci.c     | 17 +++++++++++++++--
>  lib/arm/setup.c    | 22 ++++++++++++++++++++++
>  4 files changed, 51 insertions(+), 31 deletions(-)
>
> diff --git a/arm/selftest.c b/arm/selftest.c
> index 4495b161cdd5..9f459ed3d571 100644
> --- a/arm/selftest.c
> +++ b/arm/selftest.c
> @@ -400,33 +400,13 @@ static void check_vectors(void *arg __unused)
>  	exit(report_summary());
>  }
>  
> -static bool psci_check(void)
> +static void psci_print(void)
>  {
> -	const struct fdt_property *method;
> -	int node, len, ver;
> -
> -	node = fdt_node_offset_by_compatible(dt_fdt(), -1, "arm,psci-0.2");
> -	if (node < 0) {
> -		printf("PSCI v0.2 compatibility required\n");
> -		return false;
> -	}
> -
> -	method = fdt_get_property(dt_fdt(), node, "method", &len);
> -	if (method == NULL) {
> -		printf("bad psci device tree node\n");
> -		return false;
> -	}
> -
> -	if (len < 4 || strcmp(method->data, "hvc") != 0) {
> -		printf("psci method must be hvc\n");
> -		return false;
> -	}
> -
> -	ver = psci_invoke(PSCI_0_2_FN_PSCI_VERSION, 0, 0, 0);
> -	printf("PSCI version %d.%d\n", PSCI_VERSION_MAJOR(ver),
> -				       PSCI_VERSION_MINOR(ver));
> -
> -	return true;
> +	int ver = psci_invoke(PSCI_0_2_FN_PSCI_VERSION, 0, 0, 0);
> +	report_info("PSCI version: %d.%d", PSCI_VERSION_MAJOR(ver),
> +					  PSCI_VERSION_MINOR(ver));
> +	report_info("PSCI method: %s", psci_invoke == psci_invoke_hvc ?
> +				       "hvc" : "smc");
>  }
>  
>  static void cpu_report(void *data __unused)
> @@ -465,7 +445,7 @@ int main(int argc, char **argv)
>  
>  	} else if (strcmp(argv[1], "smp") == 0) {
>  
> -		report(psci_check(), "PSCI version");
> +		psci_print();
>  		on_cpus(cpu_report, NULL);
>  		while (!cpumask_full(&ready))
>  			cpu_relax();
> diff --git a/lib/arm/asm/psci.h b/lib/arm/asm/psci.h
> index 7b956bf5987d..e385ce27f5d1 100644
> --- a/lib/arm/asm/psci.h
> +++ b/lib/arm/asm/psci.h
> @@ -3,8 +3,13 @@
>  #include <libcflat.h>
>  #include <linux/psci.h>
>  
> -extern int psci_invoke(unsigned long function_id, unsigned long arg0,
> -		       unsigned long arg1, unsigned long arg2);
> +typedef int (*psci_invoke_fn)(unsigned long function_id, unsigned long arg0,

function_id is 32bits. sizeof(unsigned long) is 64 bits for arm64.

> +			      unsigned long arg1, unsigned long arg2);
> +extern psci_invoke_fn psci_invoke;
> +extern int psci_invoke_hvc(unsigned long function_id, unsigned long arg0,
> +			   unsigned long arg1, unsigned long arg2);
> +extern int psci_invoke_smc(unsigned long function_id, unsigned long arg0,
> +			   unsigned long arg1, unsigned long arg2);
>  extern int psci_cpu_on(unsigned long cpuid, unsigned long entry_point);
>  extern void psci_system_reset(void);
>  extern int cpu_psci_cpu_boot(unsigned int cpu);
> diff --git a/lib/arm/psci.c b/lib/arm/psci.c
> index 936c83948b6a..46300f30822c 100644
> --- a/lib/arm/psci.c
> +++ b/lib/arm/psci.c
> @@ -11,9 +11,11 @@
>  #include <asm/page.h>
>  #include <asm/smp.h>
>  
> +psci_invoke_fn psci_invoke;

In setup(), we set the conduit after we call assert() several time. If the asert()
fails, then psci_system_off() will end up calling a NULL function. Maybe there
should be some sort of check for that?

> +
>  __attribute__((noinline))
> -int psci_invoke(unsigned long function_id, unsigned long arg0,
> -		unsigned long arg1, unsigned long arg2)
> +int psci_invoke_hvc(unsigned long function_id, unsigned long arg0,
> +		    unsigned long arg1, unsigned long arg2)
>  {
>  	asm volatile(
>  		"hvc #0"
> @@ -22,6 +24,17 @@ int psci_invoke(unsigned long function_id, unsigned long arg0,
>  	return function_id;
>  }
>  
> +__attribute__((noinline))
> +int psci_invoke_smc(unsigned long function_id, unsigned long arg0,
> +		    unsigned long arg1, unsigned long arg2)
> +{
> +	asm volatile(
> +		"smc #0"
> +	: "+r" (function_id)
> +	: "r" (arg0), "r" (arg1), "r" (arg2));
> +	return function_id;

I haven't been able to figure out what prevents the compiler from shuffling the
arguments around before executing the inline assembly, such that x0-x3 doesn't
contain the arguments in the order we are expecting.

Some excerpts from the extended asm help page [1] that make me believe that the
compiler doesn't provide any guarantees:

"If you must use a specific register, but your Machine Constraints do not provide
sufficient control to select the specific register you want, local register
variables may provide a solution"

"Using the generic ‘r’ constraint instead of a constraint for a specific register
allows the compiler to pick the register to use, which can result in more
efficient code."

Same with psci_invoke_hvc(). Doing both in assembly (like Linux) should be
sufficient and fairly straightforward.

[1] https://gcc.gnu.org/onlinedocs/gcc/Extended-Asm.html#Extended-Asm

> +}
> +
>  int psci_cpu_on(unsigned long cpuid, unsigned long entry_point)
>  {
>  #ifdef __arm__
> diff --git a/lib/arm/setup.c b/lib/arm/setup.c
> index 5cda2d919d2b..e595a9e5a167 100644
> --- a/lib/arm/setup.c
> +++ b/lib/arm/setup.c
> @@ -25,6 +25,7 @@
>  #include <asm/processor.h>
>  #include <asm/smp.h>
>  #include <asm/timer.h>
> +#include <asm/psci.h>
>  
>  #include "io.h"
>  
> @@ -55,6 +56,26 @@ int mpidr_to_cpu(uint64_t mpidr)
>  	return -1;
>  }
>  
> +static void psci_set_conduit(void)
> +{
> +	const void *fdt = dt_fdt();
> +	const struct fdt_property *method;
> +	int node, len;
> +
> +	node = fdt_node_offset_by_compatible(fdt, -1, "arm,psci-0.2");
> +	assert_msg(node >= 0, "PSCI v0.2 compatibility required");
> +
> +	method = fdt_get_property(fdt, node, "method", &len);
> +	assert(method != NULL && len == 4);
> +
> +	if (strcmp(method->data, "hvc") == 0)
> +		psci_invoke = psci_invoke_hvc;
> +	else if (strcmp(method->data, "smc") == 0)
> +		psci_invoke = psci_invoke_smc;
> +	else
> +		assert_msg(false, "Unknown PSCI conduit: %s", method->data);
> +}

Any particular reason for doing this here instead of in psci.c? This looks like
something that belongs to that file, but that might just be my personal preference.

Thanks,

Alex

> +
>  static void cpu_set(int fdtnode __unused, u64 regval, void *info __unused)
>  {
>  	int cpu = nr_cpus++;
> @@ -259,6 +280,7 @@ void setup(const void *fdt, phys_addr_t freemem_start)
>  	mem_regions_add_assumed();
>  	mem_init(PAGE_ALIGN((unsigned long)freemem));
>  
> +	psci_set_conduit();
>  	cpu_init();
>  
>  	/* cpu_init must be called before thread_info_init */