KVM: arm64: Avoid storing the vcpu pointer on the stack

We already have the percpu area for the host cpu state, which points to
the VCPU, so there's no need to store the VCPU pointer on the stack on
every context switch.  We can be a little more clever and just use
tpidr_el2 for the percpu offset and load the VCPU pointer from the host
context.

This has the benefit of being able to retrieve the host context even
when our stack is corrupted, and it has a potential performance benefit
because we trade a store plus a load for an mrs and a load on a round
trip to the guest.

This does require us to calculate the percpu offset without including
the offset from the kernel mapping of the percpu array to the linear
mapping of the array (which is what we store in tpidr_el1), because a
PC-relative generated address in EL2 is already giving us the hyp alias
of the linear mapping of a kernel address.  We do this in
__cpu_init_hyp_mode() by using kvm_ksym_ref().

The code that accesses ESR_EL2 was previously using an alternative to
use the _EL1 accessor on VHE systems, but this was actually unnecessary
as the _EL1 accessor aliases the ESR_EL2 register on VHE, and the _EL2
accessor does the same thing on both systems.

Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Marc Zyngier <marc.zyngier@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/arch/arm64/kernel/asm-offsets.c b/arch/arm64/kernel/asm-offsets.c
index 1303e04..78e1b0a 100644
--- a/arch/arm64/kernel/asm-offsets.c
+++ b/arch/arm64/kernel/asm-offsets.c
@@ -138,6 +138,7 @@ int main(void)
   DEFINE(CPU_FP_REGS,		offsetof(struct kvm_regs, fp_regs));
   DEFINE(VCPU_FPEXC32_EL2,	offsetof(struct kvm_vcpu, arch.ctxt.sys_regs[FPEXC32_EL2]));
   DEFINE(VCPU_HOST_CONTEXT,	offsetof(struct kvm_vcpu, arch.host_cpu_context));
+  DEFINE(HOST_CONTEXT_VCPU,	offsetof(struct kvm_cpu_context, __hyp_running_vcpu));
 #endif
 #ifdef CONFIG_CPU_PM
   DEFINE(CPU_SUSPEND_SZ,	sizeof(struct cpu_suspend_ctx));