Re: [PATCH 09/15] arm64: KVM: Simplify HYP init/teardown

Christoffer Dall
2016-06-30T13:31:39Z

On Thu, Jun 30, 2016 at 01:10:33PM +0100, Marc Zyngier wrote:
> On 28/06/16 22:31, Christoffer Dall wrote:
> > On Tue, Jun 07, 2016 at 11:58:29AM +0100, Marc Zyngier wrote:
> >> Now that we only have the "merged page tables" case to deal with,
> >> there is a bunch of things we can simplify in the HYP code (both
> >> at init and teardown time).
> >>
> >> Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
> >> ---
> >> arch/arm64/include/asm/kvm_host.h | 12 ++------
> >> arch/arm64/kvm/hyp-init.S | 61 +++++----------------------------------
> >> arch/arm64/kvm/hyp/entry.S | 19 ------------
> >> arch/arm64/kvm/hyp/hyp-entry.S | 15 ++++++++++
> >> arch/arm64/kvm/reset.c | 11 -------
> >> 5 files changed, 26 insertions(+), 92 deletions(-)
> >>
> >> diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> >> index 49095fc..88462c3 100644
> >> --- a/arch/arm64/include/asm/kvm_host.h
> >> +++ b/arch/arm64/include/asm/kvm_host.h
> >> @@ -48,7 +48,6 @@
> >> int __attribute_const__ kvm_target_cpu(void);
> >> int kvm_reset_vcpu(struct kvm_vcpu *vcpu);
> >> int kvm_arch_dev_ioctl_check_extension(long ext);
> >> -unsigned long kvm_hyp_reset_entry(void);
> >> void __extended_idmap_trampoline(phys_addr_t boot_pgd, phys_addr_t idmap_start);
> >>
> >> struct kvm_arch {
> >> @@ -357,19 +356,14 @@ static inline void __cpu_init_hyp_mode(phys_addr_t boot_pgd_ptr,
> >> * Call initialization code, and switch to the full blown
> >> * HYP code.
> >> */
> >> - __kvm_call_hyp((void *)boot_pgd_ptr, pgd_ptr,
> >> - hyp_stack_ptr, vector_ptr);
> >> + __kvm_call_hyp((void *)pgd_ptr, hyp_stack_ptr, vector_ptr);
> >> }
> >>
> >> +void __kvm_hyp_teardown(void);
> >> static inline void __cpu_reset_hyp_mode(phys_addr_t boot_pgd_ptr,
> >> phys_addr_t phys_idmap_start)
> >> {
> >> - /*
> >> - * Call reset code, and switch back to stub hyp vectors.
> >> - * Uses __kvm_call_hyp() to avoid kaslr's kvm_ksym_ref() translation.
> >> - */
> >> - __kvm_call_hyp((void *)kvm_hyp_reset_entry(),
> >> - boot_pgd_ptr, phys_idmap_start);
> >> + kvm_call_hyp(__kvm_hyp_teardown, phys_idmap_start);
> >> }
> >>
> >> static inline void kvm_arch_hardware_unsetup(void) {}
> >> diff --git a/arch/arm64/kvm/hyp-init.S b/arch/arm64/kvm/hyp-init.S
> >> index a873a6d..6b29d3d 100644
> >> --- a/arch/arm64/kvm/hyp-init.S
> >> +++ b/arch/arm64/kvm/hyp-init.S
> >> @@ -53,10 +53,9 @@ __invalid:
> >> b .
> >>
> >> /*
> >> - * x0: HYP boot pgd
> >> - * x1: HYP pgd
> >> - * x2: HYP stack
> >> - * x3: HYP vectors
> >> + * x0: HYP pgd
> >> + * x1: HYP stack
> >> + * x2: HYP vectors
> >> */
> >> __do_hyp_init:
> >>
> >> @@ -110,71 +109,27 @@ __do_hyp_init:
> >> msr sctlr_el2, x4
> >> isb
> >>
> >> - /* Skip the trampoline dance if we merged the boot and runtime PGDs */
> >> - cmp x0, x1
> >> - b.eq merged
> >> -
> >> - /* MMU is now enabled. Get ready for the trampoline dance */
> >> - ldr x4, =TRAMPOLINE_VA
> >> - adr x5, target
> >> - bfi x4, x5, #0, #PAGE_SHIFT
> >> - br x4
> >> -
> >> -target: /* We're now in the trampoline code, switch page tables */
> >> - msr ttbr0_el2, x1
> >> - isb
> >> -
> >> - /* Invalidate the old TLBs */
> >> - tlbi alle2
> >> - dsb sy
> >> -
> >> -merged:
> >> /* Set the stack and new vectors */
> >> + kern_hyp_va x1
> >> + mov sp, x1
> >> kern_hyp_va x2
> >> - mov sp, x2
> >> - kern_hyp_va x3
> >> - msr vbar_el2, x3
> >> + msr vbar_el2, x2
> >>
> >> /* Hello, World! */
> >> eret
> >> ENDPROC(__kvm_hyp_init)
> >>
> >> /*
> >> - * Reset kvm back to the hyp stub. This is the trampoline dance in
> >> - * reverse. If kvm used an extended idmap, __extended_idmap_trampoline
> >> - * calls this code directly in the idmap. In this case switching to the
> >> - * boot tables is a no-op.
> >> - *
> >> - * x0: HYP boot pgd
> >> - * x1: HYP phys_idmap_start
> >> + * Reset kvm back to the hyp stub.
> >> */
> >> ENTRY(__kvm_hyp_reset)
> >> - /* We're in trampoline code in VA, switch back to boot page tables */
> >> - msr ttbr0_el2, x0
> >> - isb
> >> -
> >> - /* Ensure the PA branch doesn't find a stale tlb entry or stale code. */
> >> - ic iallu
> >> - tlbi alle2
> >> - dsb sy
> >> - isb
> >> -
> >> - /* Branch into PA space */
> >> - adr x0, 1f
> >> - bfi x1, x0, #0, #PAGE_SHIFT
> >> - br x1
> >> -
> >> /* We're now in idmap, disable MMU */
> >> -1: mrs x0, sctlr_el2
> >> + mrs x0, sctlr_el2
> >> ldr x1, =SCTLR_ELx_FLAGS
> >> bic x0, x0, x1 // Clear SCTL_M and etc
> >> msr sctlr_el2, x0
> >> isb
> >>
> >> - /* Invalidate the old TLBs */
> >> - tlbi alle2
> >> - dsb sy
> >> -
> >
> > why can we get rid of the above two lines now?
>
> We never really needed them, as we always invalid TLBs before enabling
> the MMU. Simply disabling the MMU is enough here.
>
> >
> >> /* Install stub vectors */
> >> adr_l x0, __hyp_stub_vectors
> >> msr vbar_el2, x0
> >> diff --git a/arch/arm64/kvm/hyp/entry.S b/arch/arm64/kvm/hyp/entry.S
> >> index 70254a6..ce9e5e5 100644
> >> --- a/arch/arm64/kvm/hyp/entry.S
> >> +++ b/arch/arm64/kvm/hyp/entry.S
> >> @@ -164,22 +164,3 @@ alternative_endif
> >>
> >> eret
> >> ENDPROC(__fpsimd_guest_restore)
> >> -
> >> -/*
> >> - * When using the extended idmap, we don't have a trampoline page we can use
> >> - * while we switch pages tables during __kvm_hyp_reset. Accessing the idmap
> >> - * directly would be ideal, but if we're using the extended idmap then the
> >> - * idmap is located above HYP_PAGE_OFFSET, and the address will be masked by
> >> - * kvm_call_hyp using kern_hyp_va.
> >> - *
> >> - * x0: HYP boot pgd
> >> - * x1: HYP phys_idmap_start
> >> - */
> >> -ENTRY(__extended_idmap_trampoline)
> >> - mov x4, x1
> >> - adr_l x3, __kvm_hyp_reset
> >> -
> >> - /* insert __kvm_hyp_reset()s offset into phys_idmap_start */
> >> - bfi x4, x3, #0, #PAGE_SHIFT
> >> - br x4
> >> -ENDPROC(__extended_idmap_trampoline)
> >> diff --git a/arch/arm64/kvm/hyp/hyp-entry.S b/arch/arm64/kvm/hyp/hyp-entry.S
> >> index 2d87f36..f6d9694 100644
> >> --- a/arch/arm64/kvm/hyp/hyp-entry.S
> >> +++ b/arch/arm64/kvm/hyp/hyp-entry.S
> >> @@ -62,6 +62,21 @@ ENTRY(__vhe_hyp_call)
> >> isb
> >> ret
> >> ENDPROC(__vhe_hyp_call)
> >> +
> >> +/*
> >> + * Compute the idmap address of __kvm_hyp_reset based on the idmap
> >> + * start passed as a parameter, and jump there.
> >> + *
> >> + * x0: HYP phys_idmap_start
> >> + */
> >> +ENTRY(__kvm_hyp_teardown)
> >> + mov x4, x0
> >> + adr_l x3, __kvm_hyp_reset
> >> +
> >> + /* insert __kvm_hyp_reset()s offset into phys_idmap_start */
> >> + bfi x4, x3, #0, #PAGE_SHIFT
> >> + br x4
> >> +ENDPROC(__kvm_hyp_teardown)
> >>
> >> el1_sync: // Guest trapped into EL2
> >> save_x0_to_x3
> >> diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> >> index d044ca3..deee1b1 100644
> >> --- a/arch/arm64/kvm/reset.c
> >> +++ b/arch/arm64/kvm/reset.c
> >> @@ -132,14 +132,3 @@ int kvm_reset_vcpu(struct kvm_vcpu *vcpu)
> >> /* Reset timer */
> >> return kvm_timer_vcpu_reset(vcpu, cpu_vtimer_irq);
> >> }
> >> -
> >> -unsigned long kvm_hyp_reset_entry(void)
> >> -{
> >> - /*
> >> - * KVM is running with merged page tables, which don't have the
> >> - * trampoline page mapped. We know the idmap is still mapped,
> >> - * but can't be called into directly. Use
> >> - * __extended_idmap_trampoline to do the call.
> >> - */
> >> - return (unsigned long)kvm_ksym_ref(__extended_idmap_trampoline);
> >> -}
> >> --
> >> 2.1.4
> >>
> >
> > I'm not sure I understand why we needed the kvm_hyp_reset_entry
> > indirection before, but the resulting code here looks good to me.
>
> We still have an indirection, it is just a bit cleaner: We cannot call
> directly into the reset function located in the idmap, as the function
> is not strictly a kernel address, and the kern_hyp_va macro will mess
> with the function address. This is why we go via:
>
> __cpu_reset_hyp_mode -> __kvm_hyp_teardown -> __kvm_hyp_reset
>
> __cpu_reset_hyp_mode is the arch-agnostic entry point,
> __kvm_hyp_teardown is a normal HYP function, and __kvm_hyp_reset is the
> real thing in the idmap page.
>
> Is that clearer?
>
Didn't we have

__cpu_reset_hyp_mode -> kvm_hyp_reset_entry ->
__extended_idmap_trampoline -> __kvm_hyp_reset before, so one more level
of indirection, which we are not removing?

In any case, both versions of the code looks correct, perhaps it was
simply that kvm_hyp_reset_entry was implemented for both arm/arm64
before?

Thanks,
-Christoffer