linux-kernel - Re: [PATCH] KVM: PPC: Book3S HV: Handle non-present PTEs in page fault functions

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Message-ID: <a4e1bf29-af52-232e-d0d2-06206fa05fbe@kaod.org>
Date:   Thu, 16 Apr 2020 10:07:49 +0200
From:   Cédric Le Goater <clg@...d.org>
To:     Paul Mackerras <paulus@...abs.org>, kvm@...r.kernel.org,
        kvm-ppc@...r.kernel.org
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        groug@...d.org, David Gibson <david@...son.dropbear.id.au>
Subject: Re: [PATCH] KVM: PPC: Book3S HV: Handle non-present PTEs in page
 fault functions

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@...son.dropbear.id.au>
> Signed-off-by: Paul Mackerras <paulus@...abs.org>

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: Cédric Le Goater <clg@...d.org>

The code looks correct,

Reviewed-by: Cédric Le Goater <clg@...d.org>

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);
>