Re: [PATCH v3 29/32] KVM: arm64: Wrap the host with a stage 2

Will Deacon
2021-03-08T12:48:56Z

On Mon, Mar 08, 2021 at 09:22:29AM +0000, Quentin Perret wrote:
> On Friday 05 Mar 2021 at 19:29:06 (+0000), Will Deacon wrote:
> > On Tue, Mar 02, 2021 at 02:59:59PM +0000, Quentin Perret wrote:
> > > +static __always_inline void __load_host_stage2(void)
> > > +{
> > > + if (static_branch_likely(&kvm_protected_mode_initialized))
> > > + __load_stage2(&host_kvm.arch.mmu, host_kvm.arch.vtcr);
> > > + else
> > > + write_sysreg(0, vttbr_el2);
> >
> > I realise you've just moved the code, but why is this needed? All we've
> > done is point the stage-2 ttb at 0x0 (i.e. we haven't disabled anything).
>
> Right, but that is also used for tlb maintenance operations, e.g.
> __tlb_switch_to_host() and friends.

Good point, the VMID is needed in that situation.

> > > +int __pkvm_prot_finalize(void)
> > > +{
> > > + struct kvm_s2_mmu *mmu = &host_kvm.arch.mmu;
> > > + struct kvm_nvhe_init_params *params = this_cpu_ptr(&kvm_init_params);
> > > +
> > > + params->vttbr = kvm_get_vttbr(mmu);
> > > + params->vtcr = host_kvm.arch.vtcr;
> > > + params->hcr_el2 |= HCR_VM;
> > > + if (cpus_have_const_cap(ARM64_HAS_STAGE2_FWB))
> > > + params->hcr_el2 |= HCR_FWB;
> > > + kvm_flush_dcache_to_poc(params, sizeof(*params));
> > > +
> > > + write_sysreg(params->hcr_el2, hcr_el2);
> > > + __load_stage2(&host_kvm.arch.mmu, host_kvm.arch.vtcr);
> >
> > AFAICT, there's no ISB here. Do we need one before the TLB invalidation?
>
> You mean for the ARM64_WORKAROUND_SPECULATIVE_AT case? __load_stage2()
> should already add one for me no?

__load_stage2() _only_ has the ISB if ARM64_WORKAROUND_SPECULATIVE_AT is
present, whereas I think you need one unconditionall here so that the
system register write has taken effect before the TLB invalidation.

It's similar to the comment at the end of __tlb_switch_to_guest().

Having said that, I do worry that ARM64_WORKAROUND_SPECULATIVE_AT probably
needs a closer look in the world of pKVM, since it currently special-cases
the host.

> > > + __tlbi(vmalls12e1is);
> > > + dsb(ish);
> >
> > Given that the TLB is invalidated on the boot path, please can you add
> > a comment here about the stale entries which you need to invalidate?
>
> Sure -- that is for HCR bits cached in TLBs for VMID 0. Thinking about
> it I could probably reduce the tlbi scope as well.
>
> > Also, does this need to be inner-shareable? I thought this function ran on
> > each CPU.
>
> Hmm, correct, nsh should do.

Cool, then you can do that for both the TLBI and the DSB instructions (and
please add a comment that the invalidation is due to the HCR bits).

> > > +static void host_stage2_unmap_dev_all(void)
> > > +{
> > > + struct kvm_pgtable *pgt = &host_kvm.pgt;
> > > + struct memblock_region *reg;
> > > + u64 addr = 0;
> > > + int i;
> > > +
> > > + /* Unmap all non-memory regions to recycle the pages */
> > > + for (i = 0; i < hyp_memblock_nr; i++, addr = reg->base + reg->size) {
> > > + reg = &hyp_memory[i];
> > > + kvm_pgtable_stage2_unmap(pgt, addr, reg->base - addr);
> > > + }
> > > + kvm_pgtable_stage2_unmap(pgt, addr, ULONG_MAX);
> >
> > Is this just going to return -ERANGE?
>
> Hrmpf, yes, that wants BIT(pgt->ia_bits) I think. And that wants testing
> as well, clearly.

Agreed, maybe it's worth checking the return value.

> > > +static int host_stage2_idmap(u64 addr)
> > > +{
> > > + enum kvm_pgtable_prot prot = KVM_PGTABLE_PROT_R | KVM_PGTABLE_PROT_W;
> > > + struct kvm_mem_range range;
> > > + bool is_memory = find_mem_range(addr, &range);
> > > + struct hyp_pool *pool = is_memory ? &host_s2_mem : &host_s2_dev;
> > > + int ret;
> > > +
> > > + if (is_memory)
> > > + prot |= KVM_PGTABLE_PROT_X;
> > > +
> > > + hyp_spin_lock(&host_kvm.lock);
> > > + ret = kvm_pgtable_stage2_idmap_greedy(&host_kvm.pgt, addr, prot,
> > > + &range, pool);
> > > + if (is_memory || ret != -ENOMEM)
> > > + goto unlock;
> > > + host_stage2_unmap_dev_all();
> > > + ret = kvm_pgtable_stage2_idmap_greedy(&host_kvm.pgt, addr, prot,
> > > + &range, pool);
> >
> > I find this _really_ hard to reason about, as range is passed by reference
> > and we don't reset it after the first call returns -ENOMEM for an MMIO
> > mapping. Maybe some commentary on why it's still valid here?
>
> Sure, I'll add something. FWIW, that is intended -- -ENOMEM can only be
> caused by the call to kvm_pgtable_stage2_map() which leaves the range
> untouched. So, as long as we don't release the lock, the range returned
> by the first call to kvm_pgtable_stage2_idmap_greedy() should still be
> valid. I suppose I could call kvm_pgtable_stage2_map() directly the
> second time to make it obvious but I thought this would expose the
> internal of kvm_pgtable_stage2_idmap_greedy() a little bit too much.

I can see it both ways, but updating the kerneldoc for
kvm_pgtable_stage2_idmap_greedy() to state in which cases the range is
updated and how would be helpful. It just says "negative error code on
failure" at the moment.

Will