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SubjectRe: [RFC PATCH v2 17/26] KVM: arm64: Elevate Hyp mappings creation at EL2
On Wednesday 03 Feb 2021 at 15:31:39 (+0000), Will Deacon wrote:
> On Fri, Jan 08, 2021 at 12:15:15PM +0000, Quentin Perret wrote:
> > Previous commits have introduced infrastructure at EL2 to enable the Hyp
> > code to manage its own memory, and more specifically its stage 1 page
> > tables. However, this was preliminary work, and none of it is currently
> > in use.
> >
> > Put all of this together by elevating the hyp mappings creation at EL2
> > when memory protection is enabled. In this case, the host kernel running
> > at EL1 still creates _temporary_ Hyp mappings, only used while
> > initializing the hypervisor, but frees them right after.
> >
> > As such, all calls to create_hyp_mappings() after kvm init has finished
> > turn into hypercalls, as the host now has no 'legal' way to modify the
> > hypevisor page tables directly.
> >
> > Signed-off-by: Quentin Perret <qperret@google.com>
> > ---
> > arch/arm64/include/asm/kvm_mmu.h | 1 -
> > arch/arm64/kvm/arm.c | 62 +++++++++++++++++++++++++++++---
> > arch/arm64/kvm/mmu.c | 34 ++++++++++++++++++
> > 3 files changed, 92 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index d7ebd73ec86f..6c8466a042a9 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -309,6 +309,5 @@ static __always_inline void __load_guest_stage2(struct kvm_s2_mmu *mmu)
> > */
> > asm(ALTERNATIVE("nop", "isb", ARM64_WORKAROUND_SPECULATIVE_AT));
> > }
> > -
> > #endif /* __ASSEMBLY__ */
> > #endif /* __ARM64_KVM_MMU_H__ */
> > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > index 6af9204bcd5b..e524682c2ccf 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -1421,7 +1421,7 @@ static void cpu_prepare_hyp_mode(int cpu)
> > kvm_flush_dcache_to_poc(params, sizeof(*params));
> > }
> >
> > -static void cpu_init_hyp_mode(void)
> > +static void kvm_set_hyp_vector(void)
>
> Please do something about the naming: now we have both cpu_set_hyp_vector()
> and kvm_set_hyp_vector()!

I'll try to find something different, but no guarantees it'll be much
better :) Suggestions welcome.

> > {
> > struct kvm_nvhe_init_params *params;
> > struct arm_smccc_res res;
> > @@ -1439,6 +1439,11 @@ static void cpu_init_hyp_mode(void)
> > params = this_cpu_ptr_nvhe_sym(kvm_init_params);
> > arm_smccc_1_1_hvc(KVM_HOST_SMCCC_FUNC(__kvm_hyp_init), virt_to_phys(params), &res);
> > WARN_ON(res.a0 != SMCCC_RET_SUCCESS);
> > +}
> > +
> > +static void cpu_init_hyp_mode(void)
> > +{
> > + kvm_set_hyp_vector();
> >
> > /*
> > * Disabling SSBD on a non-VHE system requires us to enable SSBS
> > @@ -1481,7 +1486,10 @@ static void cpu_set_hyp_vector(void)
> > struct bp_hardening_data *data = this_cpu_ptr(&bp_hardening_data);
> > void *vector = hyp_spectre_vector_selector[data->slot];
> >
> > - *this_cpu_ptr_hyp_sym(kvm_hyp_vector) = (unsigned long)vector;
> > + if (!is_protected_kvm_enabled())
> > + *this_cpu_ptr_hyp_sym(kvm_hyp_vector) = (unsigned long)vector;
> > + else
> > + kvm_call_hyp_nvhe(__pkvm_cpu_set_vector, data->slot);
>
> *Very* minor nit, but it might be cleaner to have static inline functions
> with the same prototypes as the hypercalls, just to make the code even
> easier to read. e.g
>
> if (!is_protected_kvm_enabled())
> _cpu_set_vector(data->slot);
> else
> kvm_call_hyp_nvhe(__pkvm_cpu_set_vector, data->slot);
>
> you could then conceivably wrap that in a macro and avoid having the
> "is_protected_kvm_enabled()" checks explicit every time.

Happy to do this here, but are you suggesting to generalize this pattern
to other places as well?

> > }
> >
> > static void cpu_hyp_reinit(void)
> > @@ -1489,13 +1497,14 @@ static void cpu_hyp_reinit(void)
> > kvm_init_host_cpu_context(&this_cpu_ptr_hyp_sym(kvm_host_data)->host_ctxt);
> >
> > cpu_hyp_reset();
> > - cpu_set_hyp_vector();
> >
> > if (is_kernel_in_hyp_mode())
> > kvm_timer_init_vhe();
> > else
> > cpu_init_hyp_mode();
> >
> > + cpu_set_hyp_vector();
> > +
> > kvm_arm_init_debug();
> >
> > if (vgic_present)
> > @@ -1714,13 +1723,52 @@ static int copy_cpu_ftr_regs(void)
> > return 0;
> > }
> >
> > +static int kvm_hyp_enable_protection(void)
> > +{
> > + void *per_cpu_base = kvm_ksym_ref(kvm_arm_hyp_percpu_base);
> > + int ret, cpu;
> > + void *addr;
> > +
> > + if (!is_protected_kvm_enabled())
> > + return 0;
>
> Maybe I'm hung up on my previous suggestion, but I feel like we shouldn't
> get here if protected kvm isn't enabled.

The alternative is to move this check next to the call site, but it
won't help much IMO.

>
> > + if (!hyp_mem_base)
> > + return -ENOMEM;
> > +
> > + addr = phys_to_virt(hyp_mem_base);
> > + ret = create_hyp_mappings(addr, addr + hyp_mem_size - 1, PAGE_HYP);
> > + if (ret)
> > + return ret;
> > +
> > + preempt_disable();
> > + kvm_set_hyp_vector();
> > + ret = kvm_call_hyp_nvhe(__pkvm_init, hyp_mem_base, hyp_mem_size,
> > + num_possible_cpus(), kern_hyp_va(per_cpu_base));
>
> Would it make sense for the __pkvm_init() hypercall to set the vector as
> well, so that we wouldn't need to disable preemption over two hypercalls?

Not sure, kvm_set_hyp_vector() itself already does multiple hypercalls,
and I need it separate from __pkvm_init for secondary CPUs.

> Failing that, maybe move the whole preempt_disable/enable sequence into
> another function.

But that I can do.

> > + preempt_enable();
> > + if (ret)
> > + return ret;
> > +
> > + free_hyp_pgds();
> > + for_each_possible_cpu(cpu)
> > + free_page(per_cpu(kvm_arm_hyp_stack_page, cpu));
> > +
> > + return 0;
> > +}
> > +
> > /**
> > * Inits Hyp-mode on all online CPUs
> > */
> > static int init_hyp_mode(void)
> > {
> > int cpu;
> > - int err = 0;
> > + int err = -ENOMEM;
> > +
> > + /*
> > + * The protected Hyp-mode cannot be initialized if the memory pool
> > + * allocation has failed.
> > + */
> > + if (is_protected_kvm_enabled() && !hyp_mem_base)
> > + return err;
> >
> > /*
> > * Copy the required CPU feature register in their EL2 counterpart
> > @@ -1854,6 +1902,12 @@ static int init_hyp_mode(void)
> > for_each_possible_cpu(cpu)
> > cpu_prepare_hyp_mode(cpu);
> >
> > + err = kvm_hyp_enable_protection();
> > + if (err) {
> > + kvm_err("Failed to enable hyp memory protection: %d\n", err);
> > + goto out_err;
> > + }
> > +
> > return 0;
> >
> > out_err:
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 3cf9397dabdb..9d4c9251208e 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -225,15 +225,39 @@ void free_hyp_pgds(void)
> > if (hyp_pgtable) {
> > kvm_pgtable_hyp_destroy(hyp_pgtable);
> > kfree(hyp_pgtable);
> > + hyp_pgtable = NULL;
> > }
> > mutex_unlock(&kvm_hyp_pgd_mutex);
> > }
> >
> > +static bool kvm_host_owns_hyp_mappings(void)
> > +{
> > + if (static_branch_likely(&kvm_protected_mode_initialized))
> > + return false;
> > +
> > + /*
> > + * This can happen at boot time when __create_hyp_mappings() is called
> > + * after the hyp protection has been enabled, but the static key has
> > + * not been flipped yet.
> > + */
> > + if (!hyp_pgtable && is_protected_kvm_enabled())
> > + return false;
> > +
> > + BUG_ON(!hyp_pgtable);
>
> Can we fail more gracefully, e.g. by continuing without KVM?

Got any suggestion as to how that can be done? We could also just remove
that line -- that really should not happen.

Thanks!
Quentin

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