linux-kernel - Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages changed in the private fd

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Date:   Fri, 8 Apr 2022 03:06:49 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Michael Roth <michael.roth@....com>
Cc:     Chao Peng <chao.p.peng@...ux.intel.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-fsdevel@...r.kernel.org, linux-api@...r.kernel.org,
        qemu-devel@...gnu.org, Paolo Bonzini <pbonzini@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>,
        Hugh Dickins <hughd@...gle.com>,
        Jeff Layton <jlayton@...nel.org>,
        "J . Bruce Fields" <bfields@...ldses.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mike Rapoport <rppt@...nel.org>,
        Steven Price <steven.price@....com>,
        "Maciej S . Szmigiero" <mail@...iej.szmigiero.name>,
        Vlastimil Babka <vbabka@...e.cz>,
        Vishal Annapurve <vannapurve@...gle.com>,
        Yu Zhang <yu.c.zhang@...ux.intel.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        luto@...nel.org, jun.nakajima@...el.com, dave.hansen@...el.com,
        ak@...ux.intel.com, david@...hat.com
Subject: Re: [PATCH v5 11/13] KVM: Zap existing KVM mappings when pages
 changed in the private fd

On Tue, Apr 05, 2022, Michael Roth wrote:
> On Thu, Mar 10, 2022 at 10:09:09PM +0800, Chao Peng wrote:
> >  static inline bool kvm_slot_is_private(const struct kvm_memory_slot *slot)
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 67349421eae3..52319f49d58a 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -841,8 +841,43 @@ static int kvm_init_mmu_notifier(struct kvm *kvm)
> >  #endif /* CONFIG_MMU_NOTIFIER && KVM_ARCH_WANT_MMU_NOTIFIER */
> >  
> >  #ifdef CONFIG_MEMFILE_NOTIFIER
> > +static void kvm_memfile_notifier_handler(struct memfile_notifier *notifier,
> > +					 pgoff_t start, pgoff_t end)
> > +{
> > +	int idx;
> > +	struct kvm_memory_slot *slot = container_of(notifier,
> > +						    struct kvm_memory_slot,
> > +						    notifier);
> > +	struct kvm_gfn_range gfn_range = {
> > +		.slot		= slot,
> > +		.start		= start - (slot->private_offset >> PAGE_SHIFT),
> > +		.end		= end - (slot->private_offset >> PAGE_SHIFT),
> > +		.may_block 	= true,
> > +	};
> > +	struct kvm *kvm = slot->kvm;
> > +
> > +	gfn_range.start = max(gfn_range.start, slot->base_gfn);
> > +	gfn_range.end = min(gfn_range.end, slot->base_gfn + slot->npages);
> > +
> > +	if (gfn_range.start >= gfn_range.end)
> > +		return;
> > +
> > +	idx = srcu_read_lock(&kvm->srcu);
> > +	KVM_MMU_LOCK(kvm);
> > +	kvm_unmap_gfn_range(kvm, &gfn_range);
> > +	kvm_flush_remote_tlbs(kvm);
> > +	KVM_MMU_UNLOCK(kvm);
> > +	srcu_read_unlock(&kvm->srcu, idx);
> 
> Should this also invalidate gfn_to_pfn_cache mappings? Otherwise it seems
> possible the kernel might end up inadvertantly writing to now-private guest
> memory via a now-stale gfn_to_pfn_cache entry.

Yes.  Ideally we'd get these flows to share common code and avoid these goofs.
I tried very briefly but they're just different enough to make it ugly.