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Date: Wed, 20 Sep 2023 07:24:08 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: 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, 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>,
Chao Peng <chao.p.peng@...ux.intel.com>,
Fuad Tabba <tabba@...gle.com>,
Jarkko Sakkinen <jarkko@...nel.org>,
Anish Moorthy <amoorthy@...gle.com>,
Yu Zhang <yu.c.zhang@...ux.intel.com>,
Isaku Yamahata <isaku.yamahata@...el.com>,
Xu Yilun <yilun.xu@...el.com>,
Vlastimil Babka <vbabka@...e.cz>,
Vishal Annapurve <vannapurve@...gle.com>,
Ackerley Tng <ackerleytng@...gle.com>,
Maciej Szmigiero <mail@...iej.szmigiero.name>,
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 v12 14/33] KVM: Add KVM_CREATE_GUEST_MEMFD ioctl() for
guest-specific backing memory
On Tue, Sep 19, 2023, Binbin Wu wrote:
>
>
> On 9/14/2023 9:55 AM, Sean Christopherson wrote:
> [...]
> > +
> > +static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start,
> > + pgoff_t end)
> > +{
> > + struct kvm_memory_slot *slot;
> > + struct kvm *kvm = gmem->kvm;
> > + unsigned long index;
> > + bool flush = false;
> > +
> > + KVM_MMU_LOCK(kvm);
> > +
> > + kvm_mmu_invalidate_begin(kvm);
> > +
> > + xa_for_each_range(&gmem->bindings, index, slot, start, end - 1) {
> > + pgoff_t pgoff = slot->gmem.pgoff;
> > +
> > + struct kvm_gfn_range gfn_range = {
> > + .start = slot->base_gfn + max(pgoff, start) - pgoff,
> > + .end = slot->base_gfn + min(pgoff + slot->npages, end) - pgoff,
> > + .slot = slot,
> > + .may_block = true,
> > + };
> > +
> > + flush |= kvm_mmu_unmap_gfn_range(kvm, &gfn_range);
> > + }
> > +
> > + if (flush)
> > + kvm_flush_remote_tlbs(kvm);
> > +
> > + KVM_MMU_UNLOCK(kvm);
> > +}
> > +
> > +static void kvm_gmem_invalidate_end(struct kvm_gmem *gmem, pgoff_t start,
> > + pgoff_t end)
> > +{
> > + struct kvm *kvm = gmem->kvm;
> > +
> > + KVM_MMU_LOCK(kvm);
> > + if (xa_find(&gmem->bindings, &start, end - 1, XA_PRESENT))
> > + kvm_mmu_invalidate_end(kvm);
> kvm_mmu_invalidate_begin() is called unconditionally in
> kvm_gmem_invalidate_begin(),
> but kvm_mmu_invalidate_end() is not here.
> This makes the kvm_gmem_invalidate_{begin, end}() calls asymmetric.
Another ouch :-(
And there should be no need to acquire mmu_lock() unconditionally, the inode's
mutex protects the bindings, not mmu_lock.
I'll get a fix posted today. I think KVM can also add a sanity check to detect
unresolved invalidations, e.g.
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 7ba1ab1832a9..2a2d18070856 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1381,8 +1381,13 @@ static void kvm_destroy_vm(struct kvm *kvm)
* No threads can be waiting in kvm_swap_active_memslots() as the
* last reference on KVM has been dropped, but freeing
* memslots would deadlock without this manual intervention.
+ *
+ * If the count isn't unbalanced, i.e. KVM did NOT unregister between
+ * a start() and end(), then there shouldn't be any in-progress
+ * invalidations.
*/
WARN_ON(rcuwait_active(&kvm->mn_memslots_update_rcuwait));
+ WARN_ON(!kvm->mn_active_invalidate_count && kvm->mmu_invalidate_in_progress);
kvm->mn_active_invalidate_count = 0;
#else
kvm_flush_shadow_all(kvm);
or an alternative style
if (kvm->mn_active_invalidate_count)
kvm->mn_active_invalidate_count = 0;
else
WARN_ON(kvm->mmu_invalidate_in_progress)
> > + KVM_MMU_UNLOCK(kvm);
> > +}
> > +
> > +static long kvm_gmem_punch_hole(struct inode *inode, loff_t offset, loff_t len)
> > +{
> > + struct list_head *gmem_list = &inode->i_mapping->private_list;
> > + pgoff_t start = offset >> PAGE_SHIFT;
> > + pgoff_t end = (offset + len) >> PAGE_SHIFT;
> > + struct kvm_gmem *gmem;
> > +
> > + /*
> > + * Bindings must stable across invalidation to ensure the start+end
> > + * are balanced.
> > + */
> > + filemap_invalidate_lock(inode->i_mapping);
> > +
> > + list_for_each_entry(gmem, gmem_list, entry) {
> > + kvm_gmem_invalidate_begin(gmem, start, end);
> > + kvm_gmem_invalidate_end(gmem, start, end);
> > + }
> Why to loop for each gmem in gmem_list here?
>
> IIUIC, offset is the offset according to the inode, it is only meaningful to
> the inode passed in, i.e, it is only meaningful to the gmem binding with the
> inode, not others.
The code is structured to allow for multiple gmem instances per inode. This isn't
actually possible in the initial code base, but it's on the horizon[*]. I included
the list-based infrastructure in this initial series to ensure that guest_memfd
can actually support multiple files per inode, and to minimize the churn when the
"link" support comes along.
[*] https://lore.kernel.org/all/cover.1691446946.git.ackerleytng@google.com
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