The resizeable memory region / RAMBlock that is created for the cmd blob
has a maximum size of whole host pages (e.g., 4k), because RAMBlocks
work on full host pages. In addition, in i386 ACPI code:
acpi_align_size(tables->linker->cmd_blob, ACPI_BUILD_ALIGN_SIZE);
makes sure to align to multiples of 4k, padding with 0.
For example, if our cmd_blob is created with a size of 2k, the maximum
size is 4k - we cannot grow beyond that. Growing might be required
due to guest action when rebuilding the tables, but also on incoming
migration.
This automatic generation of the maximum size used to be sufficient,
however, there are cases where we cross host pages now when growing at
runtime: we exceed the maximum size of the RAMBlock and can crash QEMU when
trying to resize the resizeable memory region / RAMBlock:
$ build/qemu-system-x86_64 --enable-kvm \
-machine q35,nvdimm=on \
-smp 1 \
-cpu host \
-m size=2G,slots=8,maxmem=4G \
-object memory-backend-file,id=mem0,mem-path=/tmp/nvdimm,size=256M \
-device nvdimm,label-size=131072,memdev=mem0,id=nvdimm0,slot=1 \
-nodefaults \
-device vmgenid \
-device intel-iommu
Results in:
Unexpected error in qemu_ram_resize() at ../softmmu/physmem.c:1850:
qemu-system-x86_64: Size too large: /rom@etc/table-loader:
0x2000 > 0x1000: Invalid argument
In this configuration, we consume exactly 4k (32 entries, 128 bytes each)
when creating the VM. However, once the guest boots up and maps the MCFG,
we also create the MCFG table and end up consuming 2 additional entries
(pointer + checksum) -- which is where we try resizing the memory region
/ RAMBlock, however, the maximum size does not allow for it.
Currently, we get the following maximum sizes for our different
mutable tables based on behavior of resizeable RAMBlock:
hw table max_size
------- ---------------------------------------------------------
virt "etc/acpi/tables" ACPI_BUILD_TABLE_MAX_SIZE (0x200000)
virt "etc/table-loader" HOST_PAGE_ALIGN(initial_size)
virt "etc/acpi/rsdp" HOST_PAGE_ALIGN(initial_size)
i386 "etc/acpi/tables" ACPI_BUILD_TABLE_MAX_SIZE (0x200000)
i386 "etc/table-loader" HOST_PAGE_ALIGN(initial_size)
i386 "etc/acpi/rsdp" HOST_PAGE_ALIGN(initial_size)
microvm "etc/acpi/tables" ACPI_BUILD_TABLE_MAX_SIZE (0x200000)
microvm "etc/table-loader" HOST_PAGE_ALIGN(initial_size)
microvm "etc/acpi/rsdp" HOST_PAGE_ALIGN(initial_size)
Let's set the maximum table size for "etc/table-loader" to 64k, so we
can properly grow at runtime, which should be good enough for the future.
Migration is not concerned with the maximum size of a RAMBlock, only
with the used size - so existing setups are not affected. Of course, we
cannot migrate a VM that would have crash when started on older QEMU from
new QEMU to older QEMU without failing early on the destination when
synchronizing the RAM state:
qemu-system-x86_64: Size too large: /rom@etc/table-loader: 0x2000 > 0x1000: Invalid argument
qemu-system-x86_64: error while loading state for instance 0x0 of device 'ram'
qemu-system-x86_64: load of migration failed: Invalid argument
We'll refactor the code next, to make sure we get rid of this implicit
behavior for "etc/acpi/rsdp" as well and to make the code easier to
grasp.
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Igor Mammedov <imammedo@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>
Cc: Shannon Zhao <shannon.zhaosl@gmail.com>
Cc: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Richard Henderson <richard.henderson@linaro.org>
Cc: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210304105554.121674-2-david@redhat.com>
Reviewed-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
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