linux-kernel - Re: [PATCH 04/14] KVM: Play nice with read-only memslots when querying host page size

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Message-ID: <2c091d40-8e32-1e55-2eff-27a4b43e0674@redhat.com>
Date:   Tue, 21 Jan 2020 15:24:29 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paul Mackerras <paulus@...abs.org>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Marc Zyngier <maz@...nel.org>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        kvm-ppc@...r.kernel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        syzbot+c9d1fb51ac9d0d10c39d@...kaller.appspotmail.com,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Barret Rhoden <brho@...gle.com>,
        David Hildenbrand <david@...hat.com>,
        Jason Zeng <jason.zeng@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Liran Alon <liran.alon@...cle.com>,
        linux-nvdimm <linux-nvdimm@...ts.01.org>
Subject: Re: [PATCH 04/14] KVM: Play nice with read-only memslots when
 querying host page size

On 08/01/20 21:24, Sean Christopherson wrote:
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 5f7f06824c2b..d9aced677ddd 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -1418,15 +1418,23 @@ EXPORT_SYMBOL_GPL(kvm_is_visible_gfn);
>  
>  unsigned long kvm_host_page_size(struct kvm_vcpu *vcpu, gfn_t gfn)
>  {
> +	struct kvm_memory_slot *slot;
>  	struct vm_area_struct *vma;
>  	unsigned long addr, size;
>  
>  	size = PAGE_SIZE;
>  
> -	addr = kvm_vcpu_gfn_to_hva(vcpu, gfn);
> -	if (kvm_is_error_hva(addr))
> +	/*
> +	 * Manually do the equivalent of kvm_vcpu_gfn_to_hva() to avoid the
> +	 * "writable" check in __gfn_to_hva_many(), which will always fail on
> +	 * read-only memslots due to gfn_to_hva() assuming writes.
> +	 */
> +	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> +	if (!slot || slot->flags & KVM_MEMSLOT_INVALID)
>  		return PAGE_SIZE;
>  
> +	addr = __gfn_to_hva_memslot(slot, gfn);
> +
>  	down_read(&current->mm->mmap_sem);
>  	vma = find_vma(current->mm, addr);
>  	if (!vma)
> 

Even simpler: use kvm_vcpu_gfn_to_hva_prot

-	addr = kvm_vcpu_gfn_to_hva(vcpu, gfn);
+	addr = kvm_vcpu_gfn_to_hva_prot(vcpu, gfn, NULL);

"You are in a maze of twisty little functions, all alike".

Paolo