Re: [PATCH] KVM: PPC: Book3S HV: Handle non-present PTEs in page fault functions
From: CÃdric Le Goater
Date: Thu Apr 16 2020 - 07:57:28 EST
On 4/16/20 7:03 AM, Paul Mackerras wrote:
> Since cd758a9b57ee "KVM: PPC: Book3S HV: Use __gfn_to_pfn_memslot in HPT
> page fault handler", it's been possible in fairly rare circumstances to
> load a non-present PTE in kvmppc_book3s_hv_page_fault() when running a
> guest on a POWER8 host.
>
> Because that case wasn't checked for, we could misinterpret the non-present
> PTE as being a cache-inhibited PTE. That could mismatch with the
> corresponding hash PTE, which would cause the function to fail with -EFAULT
> a little further down. That would propagate up to the KVM_RUN ioctl()
> generally causing the KVM userspace (usually qemu) to fall over.
>
> This addresses the problem by catching that case and returning to the guest
> instead, letting it fault again, and retrying the whole page fault from
> the beginning.
>
> For completeness, this fixes the radix page fault handler in the same
> way. For radix this didn't cause any obvious misbehaviour, because we
> ended up putting the non-present PTE into the guest's partition-scoped
> page tables, leading immediately to another hypervisor data/instruction
> storage interrupt, which would go through the page fault path again
> and fix things up.
>
> Fixes: cd758a9b57ee "KVM: PPC: Book3S HV: Use __gfn_to_pfn_memslot in HPT page fault handler"
> Bugzilla: https://bugzilla.redhat.com/show_bug.cgi?id=1820402
> Reported-by: David Gibson <david@xxxxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paul Mackerras <paulus@xxxxxxxxxx>
I didn't see the reported issue with the current 5.7-rc1. Anyhow I gave
this patch a try on a P8 host and a P9 host with a radix guest and a hash
guest (using rhel6). Passthrough is fine also.
Tested-by: CÃdric Le Goater <clg@xxxxxxxx>
The code looks correct,
Reviewed-by: CÃdric Le Goater <clg@xxxxxxxx>
Thanks,
C.
> ---
> This is a reworked version of the patch David Gibson sent recently,
> with the fix applied to the radix case as well. The commit message
> is mostly stolen from David's patch.
>
> arch/powerpc/kvm/book3s_64_mmu_hv.c | 9 +++++----
> arch/powerpc/kvm/book3s_64_mmu_radix.c | 9 +++++----
> 2 files changed, 10 insertions(+), 8 deletions(-)
>
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_hv.c b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> index 3aecec8..20b7dce 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_hv.c
> @@ -604,18 +604,19 @@ int kvmppc_book3s_hv_page_fault(struct kvm_run *run, struct kvm_vcpu *vcpu,
> */
> local_irq_disable();
> ptep = __find_linux_pte(vcpu->arch.pgdir, hva, NULL, &shift);
> + pte = __pte(0);
> + if (ptep)
> + pte = *ptep;
> + local_irq_enable();
> /*
> * If the PTE disappeared temporarily due to a THP
> * collapse, just return and let the guest try again.
> */
> - if (!ptep) {
> - local_irq_enable();
> + if (!pte_present(pte)) {
> if (page)
> put_page(page);
> return RESUME_GUEST;
> }
> - pte = *ptep;
> - local_irq_enable();
> hpa = pte_pfn(pte) << PAGE_SHIFT;
> pte_size = PAGE_SIZE;
> if (shift)
> diff --git a/arch/powerpc/kvm/book3s_64_mmu_radix.c b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> index 134fbc1..7bf94ba 100644
> --- a/arch/powerpc/kvm/book3s_64_mmu_radix.c
> +++ b/arch/powerpc/kvm/book3s_64_mmu_radix.c
> @@ -815,18 +815,19 @@ int kvmppc_book3s_instantiate_page(struct kvm_vcpu *vcpu,
> */
> local_irq_disable();
> ptep = __find_linux_pte(vcpu->arch.pgdir, hva, NULL, &shift);
> + pte = __pte(0);
> + if (ptep)
> + pte = *ptep;
> + local_irq_enable();
> /*
> * If the PTE disappeared temporarily due to a THP
> * collapse, just return and let the guest try again.
> */
> - if (!ptep) {
> - local_irq_enable();
> + if (!pte_present(pte)) {
> if (page)
> put_page(page);
> return RESUME_GUEST;
> }
> - pte = *ptep;
> - local_irq_enable();
>
> /* If we're logging dirty pages, always map single pages */
> large_enable = !(memslot->flags & KVM_MEM_LOG_DIRTY_PAGES);
>