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[94.66.118.61]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-47fc88e4721sm5476884f8f.16.2026.07.30.07.35.42 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 30 Jul 2026 07:35:43 -0700 (PDT) Date: Thu, 30 Jul 2026 10:35:41 -0400 From: "Michael S. Tsirkin" To: Fabiano Rosas Cc: qemu-devel@nongnu.org, Peter Xu , Alexandr Moshkov Subject: Re: [PATCH 2/4] migration: Introduce VMStateOffset Message-ID: <20260730101353-mutt-send-email-mst@kernel.org> References: <20260729225227.1170574-1-farosas@suse.de> <20260729225227.1170574-3-farosas@suse.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260729225227.1170574-3-farosas@suse.de> Received-SPF: pass client-ip=170.10.133.124; envelope-from=mst@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -36 X-Spam_score: -3.7 X-Spam_bar: --- X-Spam_report: (-3.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.58, 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_H3=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 Wed, Jul 29, 2026 at 07:52:25PM -0300, Fabiano Rosas wrote: > When migrating a buffer or array, the vmstate code needs to know the > size of the buffer and the number of elements of the array. Today a > vmstate writer can choose from a number of macros that take as last > input the name of a struct member from where the size/num will be > read. > > At load time, the code will access those value via an opaque pointer > to the migrated data and therefore it needs to also know the size of > the struct member at that offset. > > Currently that information is communicated by means of the > VMS_VARRAY_* and VMS_VBUFFER_* flags, where each possible type is > represented by a flag. > > So far, that's all fine, but since the vmstate code makes heavy use of > macros, handling several types individually (i.e. by name: int, > int32_t, etc) requires several versions of a same macro, one for each > type. E.g: VMSTATE_VBUFFER_ALLOC_UINT32 > ^ > > This creates a pattern where the vmstate writer has to match the macro > name to the data type and has resulted in the code having a tendency > of having one macro version for each type, for each type of vmstate. > > There is also some cognitive load to deal with, e.g. > VMSTATE_VARRAY_INT32 doesn't hold an array of int32, it holds an array > of something else and the number of elements for the array is stored > in a variable of type int32. > > We're now dealing with the scenario where the code has been expecting > int32_t at some places, but a uint64_t macro variant has been added > without the code being updated. > > To address all these situations, introduce a new struct that will hold > the offset of the struct members, but also their size, so the various > extra macros can all be removed and the person writing the vmstate > doesn't need to care about type-checking. Still, keep a minimum check > that those fields are at least integers and fit into 64 bits. > > What changes: > > 1) type checking changes from individual types to a single check for > all integers; > > 2) there are new ways to access the offsets; > > num_offset -> num_offset.off > size_offset -> size_offset.off > [new] num_offset.size > [new] size_offset.size > > 2) reading the offsets goes from checking the VMS_VARRAY_* flags in an > if/elseif block to comparing offset.size against the hardcoded > sizes in bytes; > > 3) the VMS_VARRAY_* and VMS_VBUFFER_* flags become obsolete. Removed > in the next patch; > > 4) memory usage increases +1 byte per vmstate; > > Signed-off-by: Fabiano Rosas Nice work! Just some minor comments below. > --- > include/migration/vmstate.h | 66 +++++++++++++++++++++++-------------- > migration/savevm.c | 4 +-- > migration/vmstate.c | 51 ++++++++++++++++++---------- > 3 files changed, 76 insertions(+), 45 deletions(-) > > diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h > index 92a1a9fe98e..2ad3cc01371 100644 > --- a/include/migration/vmstate.h > +++ b/include/migration/vmstate.h > @@ -31,6 +31,7 @@ > > typedef struct VMStateInfo VMStateInfo; > typedef struct VMStateField VMStateField; > +typedef struct VMStateOffset VMStateOffset; > > /* > * VMStateInfo allows customized migration of objects that don't fit in > @@ -187,6 +188,11 @@ typedef enum { > MIG_PRI_MAX, > } MigrationPriority; > > +struct VMStateOffset { > + uint32_t off; > + uint8_t size; > +}; > + I think it would be great to add some comments here. What does the struct describe? size and offset of what where? And IIUC size 0 has a special meaning? And neither offset nor size ever come from the migration stream, right? > struct VMStateField { > const char *name; > size_t offset; > @@ -205,11 +211,11 @@ struct VMStateField { > * pointer point to. > */ > size_t size; > - size_t size_offset; > + VMStateOffset size_offset; > > size_t start; > int num; > - size_t num_offset; > + VMStateOffset num_offset; > const VMStateInfo *info; > enum VMStateFlags flags; > const VMStateDescription *vmsd; > @@ -328,6 +334,16 @@ extern const VMStateInfo vmstate_info_g_byte_array; > (type_check(t1, typeof_elt_of_field(t2, f)) \ > + QEMU_BUILD_BUG_ON_ZERO(!QEMU_IS_ARRAY(((t2 *)0)->f))) > > +#define type_check_int64(t) \ > + (((ptrdiff_t)0 * (ptrdiff_t)(~((t)0))) + \ > + (0 * sizeof(char[(sizeof(t) <= sizeof(uint64_t)) ? 1 : -1]))) > + > +#define vmstate_field_offset(_state, _field) { \ > + .off = (offsetof(_state, _field) + \ > + type_check_int64(typeof_field(_state, _field))), \ > + .size = sizeof(typeof_field(_state, _field)), \ > +} > + > #define vmstate_offset_value(_state, _field, _type) \ > (offsetof(_state, _field) + \ > type_check(_type, typeof_field(_state, _field))) > @@ -454,7 +470,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_INT32(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_INT32|VMS_POINTER, \ > @@ -464,7 +480,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_UINT32(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_UINT32|VMS_POINTER, \ > @@ -474,7 +490,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_INT32_ALLOC(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_INT32 | VMS_POINTER | VMS_ALLOC, \ > @@ -484,7 +500,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_UINT32_ALLOC(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_UINT32|VMS_POINTER|VMS_ALLOC, \ > @@ -494,7 +510,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_UINT16_ALLOC(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint16_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_UINT16 | VMS_POINTER | VMS_ALLOC, \ > @@ -504,7 +520,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_UINT16_UNSAFE(_field, _state, _field_num, _version, _info, _type) {\ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint16_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .size = sizeof(_type), \ > .flags = VMS_VARRAY_UINT16, \ > @@ -583,7 +599,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > _field, _state, _field_num, _version, _vmsd, _type) { \ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint8_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .vmsd = &(_vmsd), \ > .size = sizeof(_type), \ > .flags = VMS_POINTER | VMS_VARRAY_UINT8 | \ > @@ -596,7 +612,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > _field, _state, _field_num, _version, _vmsd, _type) { \ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .vmsd = &(_vmsd), \ > .size = sizeof(_type), \ > .flags = VMS_POINTER | VMS_VARRAY_UINT32 | \ > @@ -608,7 +624,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_VARRAY_OF_POINTER_UINT32(_field, _state, _field_num, _version, _info, _type) { \ > .name = (stringify(_field)), \ > .version_id = (_version), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .info = &(_info), \ > .flags = VMS_VARRAY_UINT32 | VMS_ARRAY_OF_POINTER | VMS_POINTER, \ > .offset = vmstate_offset_pointer(_state, _field, _type *), \ > @@ -650,7 +666,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > > #define VMSTATE_STRUCT_VARRAY_UINT8(_field, _state, _field_num, _version, _vmsd, _type) { \ > .name = (stringify(_field)), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint8_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .version_id = (_version), \ > .vmsd = &(_vmsd), \ > .size = sizeof(_type), \ > @@ -674,7 +690,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_STRUCT_VARRAY_POINTER_INT32(_field, _state, _field_num, _vmsd, _type) { \ > .name = (stringify(_field)), \ > .version_id = 0, \ > - .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .size = sizeof(_type), \ > .vmsd = &(_vmsd), \ > .flags = VMS_POINTER | VMS_VARRAY_INT32 | VMS_STRUCT, \ > @@ -684,7 +700,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_STRUCT_VARRAY_POINTER_UINT32(_field, _state, _field_num, _vmsd, _type) { \ > .name = (stringify(_field)), \ > .version_id = 0, \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .size = sizeof(_type), \ > .vmsd = &(_vmsd), \ > .flags = VMS_POINTER | VMS_VARRAY_INT32 | VMS_STRUCT, \ > @@ -694,7 +710,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_STRUCT_VARRAY_POINTER_UINT16(_field, _state, _field_num, _vmsd, _type) { \ > .name = (stringify(_field)), \ > .version_id = 0, \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint16_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .size = sizeof(_type), \ > .vmsd = &(_vmsd), \ > .flags = VMS_POINTER | VMS_VARRAY_UINT16 | VMS_STRUCT, \ > @@ -703,7 +719,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > > #define VMSTATE_STRUCT_VARRAY_INT32(_field, _state, _field_num, _version, _vmsd, _type) { \ > .name = (stringify(_field)), \ > - .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num, int32_t), \ > .version_id = (_version), \ > .vmsd = &(_vmsd), \ > .size = sizeof(_type), \ > @@ -713,7 +729,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > > #define VMSTATE_STRUCT_VARRAY_UINT32(_field, _state, _field_num, _version, _vmsd, _type) { \ > .name = (stringify(_field)), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .version_id = (_version), \ > .vmsd = &(_vmsd), \ > .size = sizeof(_type), \ > @@ -725,7 +741,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .vmsd = &(_vmsd), \ > - .num_offset = vmstate_offset_value(_state, _field_num, int32_t), \ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .size = sizeof(_type), \ > .flags = VMS_STRUCT|VMS_VARRAY_INT32|VMS_ALLOC|VMS_POINTER, \ > .offset = vmstate_offset_pointer(_state, _field, _type), \ > @@ -746,7 +762,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .field_exists = (_test), \ > - .size_offset = vmstate_offset_value(_state, _field_size, uint32_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .size = (_multiply), \ > .info = &vmstate_info_buffer, \ > .flags = VMS_VBUFFER|VMS_POINTER|VMS_MULTIPLY, \ > @@ -757,7 +773,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .field_exists = (_test), \ > - .size_offset = vmstate_offset_value(_state, _field_size, int32_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > .flags = VMS_VBUFFER|VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > @@ -767,7 +783,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .field_exists = (_test), \ > - .size_offset = vmstate_offset_value(_state, _field_size, uint32_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > .flags = VMS_VBUFFER|VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > @@ -777,7 +793,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .field_exists = (_test), \ > - .size_offset = vmstate_offset_value(_state, _field_size, uint64_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > .flags = VMS_VBUFFER | VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > @@ -788,7 +804,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .version_id = (_version), \ > .field_exists = (_test), \ > - .size_offset = vmstate_offset_value(_state, _field_size, uint32_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > .flags = VMS_VBUFFER|VMS_POINTER|VMS_ALLOC, \ > .offset = offsetof(_state, _field), \ > @@ -848,7 +864,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_UNUSED_VARRAY_UINT32(_state, _test, _version, _field_num, _size) {\ > .name = "unused", \ > .field_exists = (_test), \ > - .num_offset = vmstate_offset_value(_state, _field_num, uint32_t),\ > + .num_offset = vmstate_field_offset(_state, _field_num), \ > .version_id = (_version), \ > .size = (_size), \ > .info = &vmstate_info_unused_buffer, \ > @@ -862,7 +878,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .name = (stringify(_field)), \ > .field_exists = (_test), \ > .version_id = (_version), \ > - .size_offset = vmstate_offset_value(_state, _field_size, int32_t),\ > + .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_bitmap, \ > .flags = VMS_VBUFFER|VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > diff --git a/migration/savevm.c b/migration/savevm.c > index 34dd06f9f73..a272bcfd0b4 100644 > --- a/migration/savevm.c > +++ b/migration/savevm.c > @@ -873,14 +873,14 @@ static void vmstate_check(const VMStateDescription *vmsd) > * Size must be provided because dest QEMU needs that > * info to know what to allocate > */ > - assert(field->size || field->size_offset); > + assert(field->size || field->size_offset.size != 0); Why is field->size checked by coersing it to bool, but field->size_offset.size - by comparison to 0? field->size != 0 for consistency? > } else { > /* > * Otherwise size info isn't useful (because it's > * always the size of host pointer), detect accidental > * setup of sizes in this case. > */ > - assert(field->size == 0 && field->size_offset == 0); > + assert(field->size == 0 && field->size_offset.size == 0); > } > /* > * VMS_ARRAY_OF_POINTER must be used only together with one > diff --git a/migration/vmstate.c b/migration/vmstate.c > index 50ebe378452..0a0b9faa20e 100644 > --- a/migration/vmstate.c > +++ b/migration/vmstate.c > @@ -78,32 +78,45 @@ vmsd_init_ptr_marker_field(VMStateField *fake, const VMStateField *field) > }; > } > > -static int vmstate_n_elems(void *opaque, const VMStateField *field) > +static uint64_t vmstate_read_from_offset(void *opaque, > + const VMStateOffset *offset) > { > - int n_elems = 1; > + uint8_t *ptr = (uint8_t *)opaque + offset->off; > + The below can easily produce at least UB if offset is not size aligned. I *think* offset and size both come from macros that guarantee this never happens, but maybe better be safe. > + switch (offset->size) { > + case 1: > + return *(uint8_t *)ptr; > + case 2: > + return *(uint16_t *)ptr; > + case 4: > + return *(uint32_t *)ptr; > + case 8: > + return *(uint64_t *)ptr; > + } > + g_assert_not_reached(); > +} > + > +static uint64_t vmstate_n_elems(void *opaque, const VMStateField *field) > +{ > + uint64_t n_elems = 1; > > if (field->flags & VMS_ARRAY) { > n_elems = field->num; > - } else if (field->flags & VMS_VARRAY_INT32) { > - n_elems = *(int32_t *)(opaque + field->num_offset); > - } else if (field->flags & VMS_VARRAY_UINT32) { > - n_elems = *(uint32_t *)(opaque + field->num_offset); > - } else if (field->flags & VMS_VARRAY_UINT16) { > - n_elems = *(uint16_t *)(opaque + field->num_offset); > - } else if (field->flags & VMS_VARRAY_UINT8) { > - n_elems = *(uint8_t *)(opaque + field->num_offset); > + } else if (field->flags & (VMS_VARRAY_INT32 | VMS_VARRAY_UINT32 > + | VMS_VARRAY_UINT16 | VMS_VARRAY_UINT8)) { > + n_elems = vmstate_read_from_offset(opaque, &field->num_offset); > } > > trace_vmstate_n_elems(field->name, n_elems); > return n_elems; > } > > -static int vmstate_size(void *opaque, const VMStateField *field) > +static uint64_t vmstate_size(void *opaque, const VMStateField *field) > { > - int size; > + uint64_t size; > > if (field->flags & VMS_VBUFFER) { > - size = *(int32_t *)(opaque + field->size_offset); > + size = vmstate_read_from_offset(opaque, &field->size_offset); > if (field->flags & VMS_MULTIPLY) { > size *= field->size; > } > @@ -124,7 +137,7 @@ static void vmstate_handle_alloc(void *ptr, const VMStateField *field, > void *opaque) > { > if (field->flags & VMS_POINTER && field->flags & VMS_ALLOC) { > - gsize size = vmstate_size(opaque, field); > + uint64_t size = vmstate_size(opaque, field); > size *= vmstate_n_elems(opaque, field); > if (size) { > *(void **)ptr = g_malloc(size); > @@ -335,8 +348,9 @@ bool vmstate_load_vmsd(QEMUFile *f, const VMStateDescription *vmsd, > > if (exists) { > void *first_elem = opaque + field->offset; > - int i, n_elems = vmstate_n_elems(opaque, field); > - int size = vmstate_size(opaque, field); > + int i; > + uint64_t n_elems = vmstate_n_elems(opaque, field); > + uint64_t size = vmstate_size(opaque, field); > > vmstate_handle_alloc(first_elem, field, opaque); > if (field->flags & VMS_POINTER) { > @@ -650,8 +664,9 @@ static bool vmstate_save_vmsd_v(QEMUFile *f, const VMStateDescription *vmsd, > while (field->name) { > if (vmstate_field_exists(vmsd, field, opaque, version_id)) { > void *first_elem = opaque + field->offset; > - int i, n_elems = vmstate_n_elems(opaque, field); > - int size = vmstate_size(opaque, field); > + int i; > + uint64_t n_elems = vmstate_n_elems(opaque, field); > + uint64_t size = vmstate_size(opaque, field); > JSONWriter *vmdesc_loop = vmdesc; > bool is_prev_null = false; > /* > -- > 2.53.0