linux-kernel - Re: [PATCH 07/14] KVM: x86/mmu: Walk host page tables to find THP mappings

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Date:   Tue, 21 Jan 2020 15:40:43 +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 07/14] KVM: x86/mmu: Walk host page tables to find THP
 mappings

On 08/01/20 21:24, Sean Christopherson wrote:
> +
> +	/*
> +	 * 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.  Earlier
> +	 * page fault steps have already verified the guest isn't writing a
> +	 * read-only memslot.
> +	 */
> +	slot = kvm_vcpu_gfn_to_memslot(vcpu, gfn);
> +	if (!memslot_valid_for_gpte(slot, true))
> +		return PT_PAGE_TABLE_LEVEL;
> +
> +	hva = __gfn_to_hva_memslot(slot, gfn);
> +

Using gfn_to_memslot_dirty_bitmap is also a good excuse to avoid
kvm_vcpu_gfn_to_hva.

+	slot = gfn_to_memslot_dirty_bitmap(vcpu, gfn, true);
+	if (!slot)
+		return PT_PAGE_TABLE_LEVEL;
+
+	hva = __gfn_to_hva_memslot(slot, gfn);

(I am planning to remove gfn_to_hva_memslot so that __gfn_to_hva_memslot
can lose the annoying underscores).

Paolo