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AJvYcCXt1roVQ2CG5V6JW61V+oI258Hu1ismbHWPG5GiYgZFFMxpgzu5mNI2HtIuZ32sDgV3Ql+kjkfZmgCQsfg=@vger.kernel.org X-Gm-Message-State: AOJu0YyvD0qJRT9a+1KbX/Sz8pZxrn/uMsrEtUEdNnu21DAiSxLEpA92 uSWUMZNBXkiRQAbzkg0Z+LccK6gEyr1Ai24FSr2KjHQ5nWnZkqm9Tdi1aMU1UFzzyc4nzNtfYve uMDy7dQ== X-Received: from pjup23.prod.google.com ([2002:a17:90a:d317:b0:34a:b8e4:f1b9]) (user=seanjc job=prod-delivery.src-stubby-dispatcher) by 2002:a05:6a21:7741:b0:390:ca32:da20 with SMTP id adf61e73a8af0-390ca32f236mr3066283637.10.1769639960313; Wed, 28 Jan 2026 14:39:20 -0800 (PST) Date: Wed, 28 Jan 2026 14:39:18 -0800 In-Reply-To: <20260106102250.25194-1-yan.y.zhao@intel.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20260106101646.24809-1-yan.y.zhao@intel.com> <20260106102250.25194-1-yan.y.zhao@intel.com> Message-ID: Subject: Re: [PATCH v3 16/24] KVM: guest_memfd: Split for punch hole and private-to-shared conversion From: Sean Christopherson To: Yan Zhao Cc: pbonzini@redhat.com, linux-kernel@vger.kernel.org, kvm@vger.kernel.org, x86@kernel.org, rick.p.edgecombe@intel.com, dave.hansen@intel.com, kas@kernel.org, tabba@google.com, ackerleytng@google.com, michael.roth@amd.com, david@kernel.org, vannapurve@google.com, sagis@google.com, vbabka@suse.cz, thomas.lendacky@amd.com, nik.borisov@suse.com, pgonda@google.com, fan.du@intel.com, jun.miao@intel.com, francescolavra.fl@gmail.com, jgross@suse.com, ira.weiny@intel.com, isaku.yamahata@intel.com, xiaoyao.li@intel.com, kai.huang@intel.com, binbin.wu@linux.intel.com, chao.p.peng@intel.com, chao.gao@intel.com Content-Type: text/plain; charset="us-ascii" On Tue, Jan 06, 2026, Yan Zhao wrote: > virt/kvm/guest_memfd.c | 67 ++++++++++++++++++++++++++++++++++++++++++ > 1 file changed, 67 insertions(+) > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 03613b791728..8e7fbed57a20 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -486,6 +486,55 @@ static int merge_truncate_range(struct inode *inode, pgoff_t start, > return ret; > } > > +static int __kvm_gmem_split_private(struct gmem_file *f, pgoff_t start, pgoff_t end) > +{ > + enum kvm_gfn_range_filter attr_filter = KVM_FILTER_PRIVATE; > + > + bool locked = false; > + struct kvm_memory_slot *slot; > + struct kvm *kvm = f->kvm; > + unsigned long index; > + int ret = 0; > + > + xa_for_each_range(&f->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, > + .attr_filter = attr_filter, > + }; > + > + if (!locked) { > + KVM_MMU_LOCK(kvm); > + locked = true; > + } > + > + ret = kvm_split_cross_boundary_leafs(kvm, &gfn_range, false); This bleeds TDX details all over guest_memfd. Presumably SNP needs a similar callback to update the RMP, but SNP most definitely doesn't _need_ to split hugepages that now have mixed attributes. In fact, SNP can probably do literally nothing here and let kvm_gmem_zap() do the heavy lifting. Sadly, an arch hook is "necessary", because otherwise we'll end up in dependency hell. E.g. I _want_ to just let the TDP MMU do the splits during kvm_gmem_zap(), but then an -ENOMEM when splitting would result in a partial conversion if more than one KVM instance was bound to the gmem instance (ignoring that it's actually "fine" for the TDX case, because only one S-EPT tree can have a valid mapping). Even if we're willing to live with that assumption baked into the TDP MMU, we'd still need to allow kvm_gmem_zap() to fail, e.g. because -ENOMEM isn't strictly fatal. And I really, really don't want to set the precedence that "zap" operations are allow to fail. But those details absolutely do not belong in guest_memfd.c. Provide an arch hook to give x86 the opportunity to pre-split hugepages, but keep the details in arch code. static int __kvm_gmem_convert(struct gmem_file *f, pgoff_t start, pgoff_t end, bool to_private) { struct kvm_memory_slot *slot; unsigned long index; int r; xa_for_each_range(&f->bindings, index, slot, start, end - 1) { r = kvm_arch_gmem_convert(f->kvm, kvm_gmem_get_start_gfn(slot, start), kvm_gmem_get_end_gfn(slot, end), to_private); if (r) return r; } return 0; } static int kvm_gmem_convert(struct inode *inode, pgoff_t start, pgoff_t end, bool to_private) { struct gmem_file *f; int r; kvm_gmem_for_each_file(f, inode->i_mapping) { r = __kvm_gmem_convert(f, start, end, to_private); if (r) return r; } return 0; }