linux-kernel - Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report faults to userspace

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Date:   Thu, 2 Nov 2023 08:44:52 -0700
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Kai Huang <kai.huang@...el.com>, Xiaoyao Li <xiaoyao.li@...el.com>,
        "kvm-riscv@...ts.infradead.org" <kvm-riscv@...ts.infradead.org>,
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        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Isaku Yamahata <isaku.yamahata@...el.com>,
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Subject: Re: [PATCH v13 09/35] KVM: Add KVM_EXIT_MEMORY_FAULT exit to report
 faults to userspace

On Thu, Nov 02, 2023, Paolo Bonzini wrote:
> On 11/2/23 10:35, Huang, Kai wrote:
> > IIUC KVM can already handle the case of poisoned
> > page by sending signal to user app:
> > 
> > 	static int kvm_handle_error_pfn(struct kvm_vcpu *vcpu, 			struct
> > kvm_page_fault *fault)                                               	{
> > 		...
> > 
> >        		if (fault->pfn == KVM_PFN_ERR_HWPOISON) {
> >               		kvm_send_hwpoison_signal(fault->slot, fault->gfn);

No, this doesn't work, because that signals the host virtual address

	unsigned long hva = gfn_to_hva_memslot(slot, gfn);

	send_sig_mceerr(BUS_MCEERR_AR, (void __user *)hva, PAGE_SHIFT, current);

which is the *shared* page.

> >                 	return RET_PF_RETRY;
> > 	}
> > 	}
> 
> EHWPOISON is not implemented by this series, so it should be left out of the
> documentation.

EHWPOISON *is* implemented.  kvm_gmem_get_pfn() returns -EWPOISON as appropriate,
and kvm_faultin_pfn() returns that directly without going through kvm_handle_error_pfn().

  kvm_faultin_pfn_private()
  |
  |-> kvm_gmem_get_pfn()
      |
      |-> if (folio_test_hwpoison(folio)) {
		r = -EHWPOISON;
		goto out_unlock;
	  }

          |
          |-> 	r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
			     &max_order);
		if (r) {
			kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
			return r;
		}

		|
		|-> ret = __kvm_faultin_pfn(vcpu, fault);
		    if (ret != RET_PF_CONTINUE)
			    return ret;

		    if (unlikely(is_error_pfn(fault->pfn)))
			    return kvm_handle_error_pfn(vcpu, fault);