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Date: Tue, 6 Oct 2020 15:33:21 -0700
From: Ben Gardon <bgardon@...gle.com>
To: Sean Christopherson <sean.j.christopherson@...el.com>
Cc: LKML <linux-kernel@...r.kernel.org>, kvm <kvm@...r.kernel.org>,
Cannon Matthews <cannonmatthews@...gle.com>,
Paolo Bonzini <pbonzini@...hat.com>,
Peter Xu <peterx@...hat.com>, Peter Shier <pshier@...gle.com>,
Peter Feiner <pfeiner@...gle.com>,
Junaid Shahid <junaids@...gle.com>,
Jim Mattson <jmattson@...gle.com>,
Yulei Zhang <yulei.kernel@...il.com>,
Wanpeng Li <kernellwp@...il.com>,
Vitaly Kuznetsov <vkuznets@...hat.com>,
Xiao Guangrong <xiaoguangrong.eric@...il.com>
Subject: Re: [PATCH 10/22] kvm: mmu: Add TDP MMU PF handler
On Wed, Sep 30, 2020 at 9:37 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Fri, Sep 25, 2020 at 02:22:50PM -0700, Ben Gardon wrote:
> > @@ -4113,8 +4088,9 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> > if (page_fault_handle_page_track(vcpu, error_code, gfn))
> > return RET_PF_EMULATE;
> >
> > - if (fast_page_fault(vcpu, gpa, error_code))
> > - return RET_PF_RETRY;
> > + if (!is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> > + if (fast_page_fault(vcpu, gpa, error_code))
> > + return RET_PF_RETRY;
>
> It'll probably be easier to handle is_tdp_mmu() in fast_page_fault().
I'd prefer to keep this check here because then in the fast page fault
path, we can just handle the case where we do have a tdp mmu root with
the tdp mmu fast pf handler and it'll mirror the split below with
__direct_map and the TDP MMU PF handler.
>
> >
> > r = mmu_topup_memory_caches(vcpu, false);
> > if (r)
> > @@ -4139,8 +4115,14 @@ static int direct_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
> > r = make_mmu_pages_available(vcpu);
> > if (r)
> > goto out_unlock;
> > - r = __direct_map(vcpu, gpa, write, map_writable, max_level, pfn,
> > - prefault, is_tdp && lpage_disallowed);
> > +
> > + if (is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa))
> > + r = kvm_tdp_mmu_page_fault(vcpu, write, map_writable, max_level,
> > + gpa, pfn, prefault,
> > + is_tdp && lpage_disallowed);
> > + else
> > + r = __direct_map(vcpu, gpa, write, map_writable, max_level, pfn,
> > + prefault, is_tdp && lpage_disallowed);
> >
> > out_unlock:
> > spin_unlock(&vcpu->kvm->mmu_lock);
>
> ...
>
> > +/*
> > + * Handle a TDP page fault (NPT/EPT violation/misconfiguration) by installing
> > + * page tables and SPTEs to translate the faulting guest physical address.
> > + */
> > +int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu, int write, int map_writable,
> > + int max_level, gpa_t gpa, kvm_pfn_t pfn,
> > + bool prefault, bool account_disallowed_nx_lpage)
> > +{
> > + struct tdp_iter iter;
> > + struct kvm_mmu_memory_cache *pf_pt_cache =
> > + &vcpu->arch.mmu_shadow_page_cache;
> > + u64 *child_pt;
> > + u64 new_spte;
> > + int ret;
> > + int as_id = kvm_arch_vcpu_memslots_id(vcpu);
> > + gfn_t gfn = gpa >> PAGE_SHIFT;
> > + int level;
> > +
> > + if (WARN_ON(!VALID_PAGE(vcpu->arch.mmu->root_hpa)))
> > + return RET_PF_RETRY;
>
> I feel like we should kill off these silly WARNs in the existing code instead
> of adding more. If they actually fired, I'm pretty sure that they would
> continue firing and spamming the kernel log until the VM is killed as I don't
> see how restarting the guest will magically fix anything.
>
> > +
> > + if (WARN_ON(!is_tdp_mmu_root(vcpu->kvm, vcpu->arch.mmu->root_hpa)))
> > + return RET_PF_RETRY;
>
> This seems especially gratuitous, this has exactly one caller that explicitly
> checks is_tdp_mmu_root(). Again, if this fires it will spam the kernel log
> into submission.
>
> > +
> > + level = kvm_mmu_hugepage_adjust(vcpu, gfn, max_level, &pfn);
> > +
> > + for_each_tdp_pte_vcpu(iter, vcpu, gfn, gfn + 1) {
> > + disallowed_hugepage_adjust(iter.old_spte, gfn, iter.level,
> > + &pfn, &level);
> > +
> > + if (iter.level == level)
> > + break;
> > +
> > + /*
> > + * If there is an SPTE mapping a large page at a higher level
> > + * than the target, that SPTE must be cleared and replaced
> > + * with a non-leaf SPTE.
> > + */
> > + if (is_shadow_present_pte(iter.old_spte) &&
> > + is_large_pte(iter.old_spte)) {
> > + *iter.sptep = 0;
> > + handle_changed_spte(vcpu->kvm, as_id, iter.gfn,
> > + iter.old_spte, 0, iter.level);
> > + kvm_flush_remote_tlbs_with_address(vcpu->kvm, iter.gfn,
> > + KVM_PAGES_PER_HPAGE(iter.level));
> > +
> > + /*
> > + * The iter must explicitly re-read the spte here
> > + * because the new is needed before the next iteration
> > + * of the loop.
> > + */
>
> I think it'd be better to explicitly, and simply, call out that iter.old_spte
> is consumed below. It's subtle enough to warrant a comment, but the comment
> didn't actually help me. Maybe something like:
>
> /*
> * Refresh iter.old_spte, it will trigger the !present
> * path below.
> */
>
That's a good point and calling out the relation to the present check
below is much clearer.
> > + iter.old_spte = READ_ONCE(*iter.sptep);
> > + }
> > +
> > + if (!is_shadow_present_pte(iter.old_spte)) {
> > + child_pt = kvm_mmu_memory_cache_alloc(pf_pt_cache);
> > + clear_page(child_pt);
> > + new_spte = make_nonleaf_spte(child_pt,
> > + !shadow_accessed_mask);
> > +
> > + *iter.sptep = new_spte;
> > + handle_changed_spte(vcpu->kvm, as_id, iter.gfn,
> > + iter.old_spte, new_spte,
> > + iter.level);
> > + }
> > + }
> > +
> > + if (WARN_ON(iter.level != level))
> > + return RET_PF_RETRY;
>
> This also seems unnecessary. Or maybe these are all good candiates for
> KVM_BUG_ON...
>
I've replaced all these warnings with KVM_BUG_ONs.
> > +
> > + ret = page_fault_handle_target_level(vcpu, write, map_writable,
> > + as_id, &iter, pfn, prefault);
> > +
> > + /* If emulating, flush this vcpu's TLB. */
>
> Why? It's obvious _what_ the code is doing, the comment should explain _why_.
>
> > + if (ret == RET_PF_EMULATE)
> > + kvm_make_request(KVM_REQ_TLB_FLUSH, vcpu);
> > +
> > + return ret;
> > +}
> > diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> > index cb86f9fe69017..abf23dc0ab7ad 100644
> > --- a/arch/x86/kvm/mmu/tdp_mmu.h
> > +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> > @@ -14,4 +14,8 @@ void kvm_tdp_mmu_put_root_hpa(struct kvm *kvm, hpa_t root_hpa);
> >
> > bool kvm_tdp_mmu_zap_gfn_range(struct kvm *kvm, gfn_t start, gfn_t end);
> > void kvm_tdp_mmu_zap_all(struct kvm *kvm);
> > +
> > +int kvm_tdp_mmu_page_fault(struct kvm_vcpu *vcpu, int write, int map_writable,
> > + int level, gpa_t gpa, kvm_pfn_t pfn, bool prefault,
> > + bool lpage_disallowed);
> > #endif /* __KVM_X86_MMU_TDP_MMU_H */
> > --
> > 2.28.0.709.gb0816b6eb0-goog
> >
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