linux-kernel - Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn

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Message-ID: <ZLphxpSTL9Fpn1ye@yilunxu-OptiPlex-7050>
Date:   Fri, 21 Jul 2023 18:45:26 +0800
From:   Xu Yilun <yilun.xu@...el.com>
To:     Yan Zhao <yan.y.zhao@...el.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Oliver Upton <oliver.upton@...ux.dev>,
        Huacai Chen <chenhuacai@...nel.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Anup Patel <anup@...infault.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Paul Moore <paul@...l-moore.com>,
        James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>, kvm@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-mips@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        kvm-riscv@...ts.infradead.org, linux-riscv@...ts.infradead.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-security-module@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Chao Peng <chao.p.peng@...ux.intel.com>,
        Fuad Tabba <tabba@...gle.com>,
        Jarkko Sakkinen <jarkko@...nel.org>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Ackerley Tng <ackerleytng@...gle.com>,
        Maciej Szmigiero <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        David Hildenbrand <david@...hat.com>,
        Quentin Perret <qperret@...gle.com>,
        Michael Roth <michael.roth@....com>,
        Wang <wei.w.wang@...el.com>,
        Liam Merwick <liam.merwick@...cle.com>,
        Isaku Yamahata <isaku.yamahata@...il.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
Subject: Re: [RFC PATCH v11 01/29] KVM: Wrap kvm_gfn_range.pte in a
 per-action union

On 2023-07-21 at 14:26:11 +0800, Yan Zhao wrote:
> On Tue, Jul 18, 2023 at 04:44:44PM -0700, Sean Christopherson wrote:
> 
> May I know why KVM now needs to register to callback .change_pte()?

I can see the original purpose is to "setting a pte in the shadow page
table directly, instead of flushing the shadow page table entry and then
getting vmexit to set it"[1].

IIUC, KVM is expected to directly make the new pte present for new
pages in this callback, like for COW.

> As also commented in kvm_mmu_notifier_change_pte(), .change_pte() must be
> surrounded by .invalidate_range_{start,end}().
> 
> While kvm_mmu_notifier_invalidate_range_start() has called kvm_unmap_gfn_range()
> to zap all leaf SPTEs, and page fault path will not install new SPTEs
> successfully before kvm_mmu_notifier_invalidate_range_end(),
> kvm_set_spte_gfn() should not be able to find any shadow present leaf entries to
> update PFN.

I also failed to figure out how the kvm_set_spte_gfn() could pass
several !is_shadow_present_pte(iter.old_spte) check then write the new
pte.


[1] https://lore.kernel.org/all/200909222039.n8MKd4TL002696@imap1.linux-foundation.org/

Thanks,
Yilun

> 
> Or could we just delete completely
> "kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);"
> from kvm_mmu_notifier_change_pte() ?