On Mon, Sep 02, 2019 at 03:55:48PM +0100, Alexandru Elisei wrote: > On 8/28/19 4:14 PM, Alexandru Elisei wrote: > > > On 8/28/19 3:45 PM, Andrew Jones wrote: > >> On Wed, Aug 28, 2019 at 02:38:17PM +0100, Alexandru Elisei wrote: > >>> 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. 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 114726feab82..5d4fe4b1570b 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/ptrace.h> > >>> @@ -138,6 +139,12 @@ 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 halt > >> Shouldn't we load PSCI_POWER_STATE_TYPE_POWER_DOWN into r1 and > >> zero out r2 and r3, as cpu_psci_cpu_die() does? And maybe we > >> should just do a 'b .' here instead of 'b halt' in order to > >> avoid confusion as to how we ended up in halt(), if the psci > >> invocation were to ever fail. > > Not really, I'm not really sure where the extra parameter in cpu_psci_cpu_die > > comes from. I have looked at ARM DEN 0022D, section 5.1.3, and the CPU_OFFcall > > has exactly one parameter, the function id. I've also looked at how KVM handles > > this call, and it doesn't use anything else other than the function id. Please > > correct me if I missed something. > > Did some digging, apparently the power state parameter was required for the very > first version of PSCI. ARM DEN 0022D states that it has been removed in PSCIv0.2: > > "7.3 Changes in PSCIv0.2 from first proposal > > [..] > > Removed power_state parameter for CPU_OFF." > > The kvm-unit-tests implementation of psci uses fixed function ids (as opposed to > first psci version, where the ids were taken from the DT), so I think that we > can drop the PSCI_POWER_STATE_TYPE_POWER_DOWN parameter in cpu_psci_cpu_die > altogether. What do you think? Sounds good to me. Thanks for the digging. drew > > Thanks, > Alex > > As for zero'ing the extra registers, this is not part of the SMC calling > > convention, this is just something that the C code for psci does. The SMC > > calling convention states that registers 0-3 will be modified after the call, so > > having 4 arguments to the psci_invoke function will tell the compiler to > > save/restore the registers in the caller. > > > > As for doing 'b .' instead of branching to halt, that's a good idea, I'll do > > that. But it will only be useful if the last CPU_OFF call fails. > > > > Thanks, > > Alex
