On Tue, 31 Dec 2019 16:09:37 +0000
Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote:
Hi,
> The psci test performs a series of CPU_ON/CPU_OFF cycles for CPU 1. This is
> done by setting the entry point for the CPU_ON call to the physical address
> of the C function cpu_psci_cpu_die.
>
> The compiler is well within its rights to use the stack when generating
> code for cpu_psci_cpu_die.
I am a bit puzzled: Is this an actual test failure at the moment? Or just a potential problem? Because I see it using the stack pointer in the generated code in lib/arm/psci.o. But the psci test seems to pass. Or is that just because the SP is somehow not cleared, because of some KVM implementation specifics?
One more thing below ...
> However, because no stack initialization has
> been done, the stack pointer is zero, as set by KVM when creating the VCPU.
> This causes a data abort without a change in exception level. The VBAR_EL1
> register is also zero (the KVM reset value for VBAR_EL1), the MMU is off,
> and we end up trying to fetch instructions from address 0x200.
>
> At this point, a stage 2 instruction abort is generated which is taken to
> KVM. KVM interprets this as an instruction fetch from an I/O region, and
> injects a prefetch abort into the guest. Prefetch abort is a synchronous
> exception, and on guest return the VCPU PC will be set to VBAR_EL1 + 0x200,
> which is... 0x200. The VCPU ends up in an infinite loop causing a prefetch
> abort while fetching the instruction to service the said abort.
>
> cpu_psci_cpu_die is basically a wrapper over the HVC instruction, so
> provide an assembly implementation for the function which will serve as the
> entry point for CPU_ON.
>
> Signed-off-by: Alexandru Elisei <alexandru.elisei@xxxxxxx>
> ---
> arm/cstart.S | 7 +++++++
> arm/cstart64.S | 7 +++++++
> arm/psci.c | 5 +++--
> 3 files changed, 17 insertions(+), 2 deletions(-)
>
> diff --git a/arm/cstart.S b/arm/cstart.S
> index 2c81d39a666b..dfef48e4dbb2 100644
> --- a/arm/cstart.S
> +++ b/arm/cstart.S
> @@ -7,6 +7,7 @@
> */
> #define __ASSEMBLY__
> #include <auxinfo.h>
> +#include <linux/psci.h>
> #include <asm/thread_info.h>
> #include <asm/asm-offsets.h>
> #include <asm/pgtable-hwdef.h>
> @@ -139,6 +140,12 @@ secondary_entry:
> blx r0
> b do_idle
>
> +.global asm_cpu_psci_cpu_die
> +asm_cpu_psci_cpu_die:
> + ldr r0, =PSCI_0_2_FN_CPU_OFF
> + hvc #0
> + b .
I am wondering if this implementation is actually too simple. Both the current implementation and the kernel clear at least the first three arguments to 0.
I failed to find a requirement for doing this (nothing in the SMCCC or the PSCI spec), but I guess it would make sense when looking at forward compatibility.
At the very least it's a change in behaviour (ignoring the missing printf).
So shall we just clear r1, r2 and r3 here? (Same for arm64 below)
Cheers,
Andre
> +
> .globl halt
> halt:
> 1: wfi
> diff --git a/arm/cstart64.S b/arm/cstart64.S
> index b0e8baa1a23a..c98842f11e90 100644
> --- a/arm/cstart64.S
> +++ b/arm/cstart64.S
> @@ -7,6 +7,7 @@
> */
> #define __ASSEMBLY__
> #include <auxinfo.h>
> +#include <linux/psci.h>
> #include <asm/asm-offsets.h>
> #include <asm/ptrace.h>
> #include <asm/processor.h>
> @@ -128,6 +129,12 @@ secondary_entry:
> blr x0
> b do_idle
>
> +.globl asm_cpu_psci_cpu_die
> +asm_cpu_psci_cpu_die:
> + ldr x0, =PSCI_0_2_FN_CPU_OFF
> + hvc #0
> + b .
> +
> .globl halt
> halt:
> 1: wfi
> diff --git a/arm/psci.c b/arm/psci.c
> index 5c1accb6cea4..c45a39c7d6e8 100644
> --- a/arm/psci.c
> +++ b/arm/psci.c
> @@ -72,6 +72,7 @@ static int cpu_on_ret[NR_CPUS];
> static cpumask_t cpu_on_ready, cpu_on_done;
> static volatile int cpu_on_start;
>
> +extern void asm_cpu_psci_cpu_die(void);
> static void cpu_on_secondary_entry(void)
> {
> int cpu = smp_processor_id();
> @@ -79,7 +80,7 @@ static void cpu_on_secondary_entry(void)
> cpumask_set_cpu(cpu, &cpu_on_ready);
> while (!cpu_on_start)
> cpu_relax();
> - cpu_on_ret[cpu] = psci_cpu_on(cpus[1], __pa(cpu_psci_cpu_die));
> + cpu_on_ret[cpu] = psci_cpu_on(cpus[1], __pa(asm_cpu_psci_cpu_die));
> cpumask_set_cpu(cpu, &cpu_on_done);
> }
>
> @@ -104,7 +105,7 @@ static bool psci_cpu_on_test(void)
> cpu_on_start = 1;
> smp_mb();
>
> - cpu_on_ret[0] = psci_cpu_on(cpus[1], __pa(cpu_psci_cpu_die));
> + cpu_on_ret[0] = psci_cpu_on(cpus[1], __pa(asm_cpu_psci_cpu_die));
> cpumask_set_cpu(0, &cpu_on_done);
>
> while (!cpumask_full(&cpu_on_done))
