Re: [PATCH 2/4] migration: Introduce VMStateOffset
Fabiano Rosas <[email protected]> Fri, 31 Jul 2026 13:19:38 -0300
| Newsgroups | org.nongnu.qemu-devel |
|---|---|
| Message-ID | <[email protected]> |
Peter Xu <[email protected]> writes: > On Fri, Jul 31, 2026 at 11:02:27AM -0300, Fabiano Rosas wrote: >> Hi Vladimir, I've been looking at this, could you clarify which vmstates >> do you think we could merge? I don't see it, either VARRAY vs. VBUFFER >> or VARRAY vs. BUFFER, also ARRAY vs. BUFFER doesn't seem to work. >> >> One main point of difference is the size_offset/num_offset variants are >> only known at load-time, so we can't convert them between each other at >> build time because the either the total size or num will not be know. > > Something like this? > > NOTE: I hid a fix to a comment that is irrelevant.. which I mentioned @size > and @size_offset can't co-exist, but it can when VMS_MULTIPLY is set. I > had a vague feeling we can further clean vmstate flags. Is there a reason we cannot use .num for MULTIPLY instead of .size? > > diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h > index 24631fd678..3a02709650 100644 > --- a/include/migration/vmstate.h > +++ b/include/migration/vmstate.h > @@ -69,17 +69,15 @@ enum VMStateFlags { > * }). Dereference the pointer before using it as basis for > * further pointer arithmetic (see e.g. VMS_ARRAY). Does not > * affect the meaning of VMStateField.num_offset or > - * VMStateField.size_offset; see VMS_VARRAY and VMS_VBUFFER for > - * those. > + * VMStateField.size_offset; see VMS_VARRAY for those. > */ > VMS_POINTER = 0x002, > > /* The field is an array of fixed size. VMStateField.num contains > * the number of entries in the array. The size of each entry is > * given by VMStateField.size and / or opaque + > - * VMStateField.size_offset; see VMS_VBUFFER and > - * VMS_MULTIPLY. Each array entry will be processed individually > - * (VMStateField.info.get()/put() if VMS_STRUCT is not set, > + * VMStateField.size_offset. Each array entry will be processed > + * individually (VMStateField.info.get()/put() if VMS_STRUCT is not set, > * recursion into VMStateField.vmsd if VMS_STRUCT is set). May not > * be combined with VMS_VARRAY. > */ > @@ -108,18 +106,10 @@ enum VMStateFlags { > */ > VMS_ARRAY_OF_POINTER = 0x040, > > - /* The size of the individual entries (a single array entry if > - * VMS_ARRAY or VMS_VARRAY are set, or the field itself if > - * neither is set) is variable (i.e. not known at compile-time), > - * but the same for all entries. Use the int32_t at opaque + > - * VMStateField.size_offset (subject to VMS_MULTIPLY) to determine > - * the size of each (and every) entry. */ > - VMS_VBUFFER = 0x100, > - > /* Multiply the entry size given by the int32_t at opaque + > - * VMStateField.size_offset (see VMS_VBUFFER description) with > - * VMStateField.size to determine the number of bytes to be > - * allocated. Only valid in combination with VMS_VBUFFER. */ > + * VMStateField.size_offset with VMStateField.size to determine the > + * number of bytes to be allocated. > + */ > VMS_MULTIPLY = 0x200, > > /* Fail loading the serialised VM state if this field is missing > @@ -181,8 +171,8 @@ struct VMStateField { > > /* > * @size or @size_offset specifies the size of the element embeded in > - * the field. Only one of them should be present never both. When > - * @size_offset is used together with VMS_VBUFFER, it means the size is > + * the field. Only one of them should be present never both, except > + * VMS_MULTIPLY. When @size_offset is used, it means the size is > * dynamic calculated instead of a constant. > * > * When the field is an array of any type, this stores the size of one > @@ -696,7 +686,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .size_offset = vmstate_field_offset(_state, _field_size), \ > .size = (_multiply), \ > .info = &vmstate_info_buffer, \ > - .flags = VMS_VBUFFER|VMS_POINTER|VMS_MULTIPLY, \ > + .flags = VMS_VARRAY|VMS_POINTER|VMS_MULTIPLY, \ > .offset = offsetof(_state, _field), \ > } > > @@ -706,7 +696,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .field_exists = (_test), \ > .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > - .flags = VMS_VBUFFER|VMS_POINTER, \ > + .flags = VMS_VARRAY|VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > } > > @@ -720,7 +710,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .field_exists = (_test), \ > .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_buffer, \ > - .flags = VMS_VBUFFER|VMS_POINTER|VMS_ALLOC, \ > + .flags = VMS_VARRAY|VMS_POINTER|VMS_ALLOC, \ > .offset = offsetof(_state, _field), \ > } > > @@ -798,7 +788,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; > .version_id = (_version), \ > .size_offset = vmstate_field_offset(_state, _field_size), \ > .info = &vmstate_info_bitmap, \ > - .flags = VMS_VBUFFER|VMS_POINTER, \ > + .flags = VMS_VARRAY|VMS_POINTER, \ > .offset = offsetof(_state, _field), \ > } > > @@ -1200,9 +1190,6 @@ extern const VMStateInfo vmstate_info_g_byte_array; > #define VMSTATE_BUFFER_START_MIDDLE(_f, _s, _start) \ > VMSTATE_BUFFER_START_MIDDLE_V(_f, _s, _start, 0) > > -#define VMSTATE_PARTIAL_VBUFFER(_f, _s, _size) \ > - VMSTATE_VBUFFER(_f, _s, 0, NULL, _size) > - > #define VMSTATE_PARTIAL_VBUFFER_UINT32(_f, _s, _size) \ > VMSTATE_VBUFFER_UINT32(_f, _s, 0, NULL, _size) > > diff --git a/migration/vmstate.c b/migration/vmstate.c > index bc8eb3d3ca..f7363a8776 100644 > --- a/migration/vmstate.c > +++ b/migration/vmstate.c > @@ -102,7 +102,7 @@ static uint64_t vmstate_n_elems(void *opaque, const VMStateField *field) > > if (field->flags & VMS_ARRAY) { > n_elems = field->num; > - } else if (field->flags & VMS_VARRAY) { > + } else if (field->num_offset.size) { Hmm, is it better to infer from the data rather than having the explicit flag? And if so, can't we remove VMS_VARRAY flag altogether? From Vladimir's message I though the intent was not only to remove the flag, but to remove the definition of VMSTATE_VBUFFER entirely and somehow fit that case into VMSTATE_VARRAY, removing size_offset along with it. > n_elems = vmstate_read_from_offset(opaque, > &field->num_offset); } > > @@ -114,17 +114,17 @@ static uint64_t vmstate_size(void *opaque, const VMStateField *field) > { > uint64_t size; > > - if (field->flags & VMS_VBUFFER) { > - size = vmstate_read_from_offset(opaque, &field->size_offset); > - if (field->flags & VMS_MULTIPLY) { > - size *= field->size; > - } > - } else if (field->flags & VMS_ARRAY_OF_POINTER) { > + if (field->flags & VMS_ARRAY_OF_POINTER) { > /* > * For an array of pointer, the each element is always size of a > * host pointer. > */ > size = sizeof(void *); > + } else if (field->size_offset.size) { > + size = vmstate_read_from_offset(opaque, &field->size_offset); > + if (field->flags & VMS_MULTIPLY) { > + size *= field->size; > + } > } else { > size = field->size; Since we're talking about flags, I'd like to be able to assert(size) at this point, but there is some weirness with vmstate_msix and vmstate_scsi_device that have no state at all. I'm thinking of introducing a new flag to identify those cases: -- >8 -- From: Fabiano Rosas <[email protected]> Subject: [PATCH] migration: Add VMS_NO_STATE flag There are a few special cases of vmstate usage: The vmstate_msix and vmstate_scsi_device have fields that contain no data, only a vmstate_info structure. The VMSTATE_VALIDATE macro serves only to invoke the .field_exists routine for validation. Regardless whether these scenarios are valid, add a separate flag to identify them so we can enforce common constraints for the normal vmstates such as having a size greater than zero. Signed-off-by: Fabiano Rosas <[email protected]> --- hw/pci/msix.c | 6 +----- hw/scsi/scsi-bus.c | 6 +----- include/migration/vmstate.h | 9 +++++++-- migration/savevm.c | 3 +-- 4 files changed, 10 insertions(+), 14 deletions(-) diff --git a/hw/pci/msix.c b/hw/pci/msix.c index 1b23eaf1007..adf76b5bccc 100644 --- a/hw/pci/msix.c +++ b/hw/pci/msix.c @@ -711,12 +711,8 @@ const VMStateDescription vmstate_msix = { .fields = (const VMStateField[]) { { .name = "msix", - .version_id = 0, - .field_exists = NULL, - .size = 0, /* ouch */ .info = &vmstate_info_msix, - .flags = VMS_SINGLE, - .offset = 0, + .flags = VMS_SINGLE | VMS_NO_STATE, }, VMSTATE_END_OF_LIST() } diff --git a/hw/scsi/scsi-bus.c b/hw/scsi/scsi-bus.c index deb43d5560e..aa02ff631b7 100644 --- a/hw/scsi/scsi-bus.c +++ b/hw/scsi/scsi-bus.c @@ -1980,12 +1980,8 @@ const VMStateDescription vmstate_scsi_device = { VMSTATE_UINT32(sense_len, SCSIDevice), { .name = "requests", - .version_id = 0, - .field_exists = NULL, - .size = 0, /* ouch */ .info = &vmstate_info_scsi_requests, - .flags = VMS_SINGLE, - .offset = 0, + .flags = VMS_SINGLE | VMS_NO_STATE, }, VMSTATE_END_OF_LIST() }, diff --git a/include/migration/vmstate.h b/include/migration/vmstate.h index 09b2e535645..ada1625f5b6 100644 --- a/include/migration/vmstate.h +++ b/include/migration/vmstate.h @@ -108,6 +108,12 @@ enum VMStateFlags { */ VMS_ARRAY_OF_POINTER = 0x040, + /* + * The field contains no data. Used for special cases such as + * invoking a custom VMStateInfo. + */ + VMS_NO_STATE = 0x080, + /* The size of the individual entries (a single array entry if * VMS_ARRAY or VMS_VARRAY are set, or the field itself if * neither is set) is variable (i.e. not known at compile-time), @@ -451,8 +457,7 @@ extern const VMStateInfo vmstate_info_g_byte_array; #define VMSTATE_VALIDATE(_name, _test) { \ .name = (_name), \ .field_exists = (_test), \ - .flags = VMS_ARRAY | VMS_MUST_EXIST, \ - .num = 0, /* 0 elements: no data, only run _test */ \ + .flags = VMS_ARRAY | VMS_MUST_EXIST | VMS_NO_STATE, \ } #define VMSTATE_POINTER(_field, _state, _version, _info, _type) { \ diff --git a/migration/savevm.c b/migration/savevm.c index 3e5cce6520d..160c0f2d97d 100644 --- a/migration/savevm.c +++ b/migration/savevm.c @@ -617,8 +617,7 @@ static void dump_vmstate_vmsd(FILE *out_file, fprintf(out_file, ",\n%*s\"Fields\": [\n", indent, ""); first = true; while (field->name != NULL) { - if (field->flags & VMS_MUST_EXIST) { - /* Ignore VMSTATE_VALIDATE bits; these don't get migrated */ + if (field->flags & VMS_NO_STATE) { field++; continue; } -- 2.53.0