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Wed, 13 May 2026 13:47:45 -0700 (PDT) X-Received: by 2002:a05:622a:17c6:b0:50d:8c24:20f2 with SMTP id d75a77b69052e-5162f574f4emr65271411cf.30.1778705264695; Wed, 13 May 2026 13:47:44 -0700 (PDT) Received: from x1.local ([142.189.10.167]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8c90c358539sm5514876d6.41.2026.05.13.13.47.43 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 13 May 2026 13:47:44 -0700 (PDT) Date: Wed, 13 May 2026 16:47:43 -0400 From: Peter Xu To: =?utf-8?Q?Marc-Andr=C3=A9?= Lureau Cc: qemu-devel@nongnu.org, Paolo Bonzini , Chenyi Qiang Subject: Re: [PATCH v4 13/13] RFC: hw/virtio: start virtio-mem guest_memfd regions as shared Message-ID: References: <20260504-rdm5-v4-0-bdf61e57c1e1@redhat.com> <20260504-rdm5-v4-13-bdf61e57c1e1@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <20260504-rdm5-v4-13-bdf61e57c1e1@redhat.com> Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -24 X-Spam_score: -2.5 X-Spam_bar: -- X-Spam_report: (-2.5 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.445, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_PASS=-0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: qemu development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org On Mon, May 04, 2026 at 04:30:19PM +0400, Marc-André Lureau wrote: > In TDX guests, virtio-mem plug/unplug/re-plug fails because > kvm_set_phys_mem() unconditionally sets KVM memory attributes to > PRIVATE for all guest_memfd regions. On re-plug, the PRIVATE->PRIVATE > transition is a no-op, so KVM doesn't re-AUG pages and the guest's > TDG.MEM.PAGE.ACCEPT fails. Know little on TDX, please bare with me.. I saw KVM does a seamcall to ADD or AUG whenever a new EPT pte is set, via this path: __tdp_mmu_set_spte_atomic set_external_spte_present tdx_sept_set_private_spte <------ On unplug, I'm expecting with your prior patches, gmem pages will be truncated properly, so they'll be all gone. Then, qemu does replug -> guest gets that event, start access page -> EPT violation, KVM resolving page fault with __tdp_mmu_set_spte_atomic() (per above) and a new page -> triggering AUG (not ADD, since it's post-boot). Could you elaborate here why AUG is missing in the first place? Thanks, > > Implement the "start-shared" approach: virtio-mem memory starts with > shared KVM attributes. The guest converts shared->private on plug (via > set_memory_encrypted -> MapGPA + ACCEPT), and back to shared on unplug > (via set_memory_decrypted). This ensures every plug triggers a real > SHARED->PRIVATE transition, causing KVM to AUG fresh pages. > > Add RAM_GUEST_MEMFD_START_SHARED flag and set it during virtio-mem > realize for guest_memfd-backed regions. Use > ram_block_attributes_state_change() to properly update the attributes > bitmap through the API. Skip setting PRIVATE in kvm_set_phys_mem() > when the flag is set. On unplug, explicitly reset KVM attributes to > shared on the host side to handle the case where the guest skips > set_memory_decrypted(). > > See also virtio-comment "[PATCH RFC] virtio-mem: add shared/private memory property details". > > Signed-off-by: Marc-André Lureau > --- > include/system/memory.h | 6 ++++++ > accel/kvm/kvm-all.c | 3 ++- > hw/virtio/virtio-mem.c | 27 ++++++++++++++++++++++++++- > 3 files changed, 34 insertions(+), 2 deletions(-) > > diff --git a/include/system/memory.h b/include/system/memory.h > index 28a75dac4ae..9dbf67efe50 100644 > --- a/include/system/memory.h > +++ b/include/system/memory.h > @@ -277,6 +277,12 @@ typedef struct IOMMUTLBEvent { > */ > #define RAM_PRIVATE (1 << 13) > > +/* > + * RAM with guest_memfd that should start with shared KVM memory > + * attributes. The guest converts to private on use. > + */ > +#define RAM_GUEST_MEMFD_START_SHARED (1 << 14) > + > static inline void iommu_notifier_init(IOMMUNotifier *n, IOMMUNotify fn, > IOMMUNotifierFlag flags, > hwaddr start, hwaddr end, > diff --git a/accel/kvm/kvm-all.c b/accel/kvm/kvm-all.c > index 97463a683f4..c034e74c8e5 100644 > --- a/accel/kvm/kvm-all.c > +++ b/accel/kvm/kvm-all.c > @@ -1737,7 +1737,8 @@ static void kvm_set_phys_mem(KVMMemoryListener *kml, > abort(); > } > > - if (memory_region_has_guest_memfd(mr)) { > + if (memory_region_has_guest_memfd(mr) && > + !(mr->ram_block->flags & RAM_GUEST_MEMFD_START_SHARED)) { > err = kvm_set_memory_attributes_private(start_addr, slot_size); > if (err) { > error_report("%s: failed to set memory attribute private: %s", > diff --git a/hw/virtio/virtio-mem.c b/hw/virtio/virtio-mem.c > index 35e03ed7599..b46efe21126 100644 > --- a/hw/virtio/virtio-mem.c > +++ b/hw/virtio/virtio-mem.c > @@ -19,6 +19,7 @@ > #include "system/memory.h" > #include "system/numa.h" > #include "system/system.h" > +#include "system/kvm.h" > #include "system/ramblock.h" > #include "system/reset.h" > #include "system/runstate.h" > @@ -479,6 +480,11 @@ static int virtio_mem_set_block_state(VirtIOMEM *vmem, uint64_t start_gpa, > if (vmem->dynamic_memslots) { > virtio_mem_deactivate_unplugged_memslots(vmem, offset, size); > } > + if (rb->flags & RAM_GUEST_MEMFD_START_SHARED) { > + kvm_set_memory_attributes_shared(start_gpa, size); > + ram_block_attributes_state_change(rb->attributes, > + offset, size, false); > + } > return 0; > } > > @@ -606,10 +612,12 @@ static int virtio_mem_unplug_all(VirtIOMEM *vmem) > RAMBlock *rb = vmem->memdev->mr.ram_block; > > if (vmem->size) { > + uint64_t used = qemu_ram_get_used_length(rb); > + > if (virtio_mem_is_busy()) { > return -EBUSY; > } > - if (ram_block_discard_range(rb, 0, qemu_ram_get_used_length(rb))) { > + if (ram_block_discard_range(rb, 0, used)) { > return -EBUSY; > } > virtio_mem_notify_unplug_all(vmem); > @@ -622,6 +630,11 @@ static int virtio_mem_unplug_all(VirtIOMEM *vmem) > if (vmem->dynamic_memslots) { > virtio_mem_deactivate_unplugged_memslots(vmem, 0, region_size); > } > + if (rb->flags & RAM_GUEST_MEMFD_START_SHARED) { > + kvm_set_memory_attributes_shared(vmem->addr, used); > + ram_block_attributes_state_change(rb->attributes, > + 0, used, false); > + } > } > > trace_virtio_mem_unplugged_all(); > @@ -859,6 +872,18 @@ static void virtio_mem_device_realize(DeviceState *dev, Error **errp) > rb = vmem->memdev->mr.ram_block; > page_size = qemu_ram_pagesize(rb); > > + /* > + * For CoCo VMs with guest_memfd, use the "start-shared" model: > + * memory starts as shared and the guest converts to private on > + * plug. > + */ > + if (rb->flags & RAM_GUEST_MEMFD) { > + rb->flags |= RAM_GUEST_MEMFD_START_SHARED; > + ram_block_attributes_state_change(rb->attributes, 0, > + qemu_ram_get_used_length(rb), > + false); > + } > + > if (virtio_mem_has_legacy_guests()) { > switch (vmem->unplugged_inaccessible) { > case ON_OFF_AUTO_AUTO: > > -- > 2.54.0 > > -- Peter Xu