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Tsirkin" , Richard Henderson , qemu-devel@nongnu.org, Alistair Francis , Shannon Zhao , qemu-arm@nongnu.org, Paolo Bonzini Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" On 02.03.21 19:43, David Hildenbrand wrote: >>>> The resizeable memory region that is created for the cmd blob has a maximum >>>> size of ACPI_BUILD_ALIGN_SIZE - 4k. This used to be sufficient, however, >> >> The expression "ACPI_BUILD_ALIGN_SIZE - 4k" makes no sense to me. >> ACPI_BUILD_ALIGN_SIZE is #defined in "hw/i386/acpi-build.c" as 0x1000, >> so the difference (ACPI_BUILD_ALIGN_SIZE - 4k) is zero. >> >> (1) Did you mean "ACPI_BUILD_ALIGN_SIZE -- 4k"? IOW, did you mean to >> quote the value of the macro? >> >> If you mean an em dash, then please use an em dash, not a hyphen (or >> please use parens). > > Yes, or rather use ACPI_BUILD_ALIGN_SIZE (4k). > >> >> >>>> as we try fitting in additional data (e.g., vmgenid, nvdimm, intel-iommu), >>>> we require more than 4k and can crash QEMU when trying to resize the >>>> resizeable memory region beyond its maximum size: >>>> $ 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 >>>> >>>> We try growing the resizeable memory region (resizeable RAMBlock) beyond >>>> its maximum size. Let's increase the maximum size from 4k to 64k, which >>>> should be good enough for the near future. >> >> The existent code calls acpi_align_size(), for resizing the ACPI blobs >> (the GArray objects). >> >> (Unfortunately, the acpi_align_size() function is duplicated between >> "hw/i386/acpi-build.c" and "hw/arm/virt-acpi-build.c", which seems >> unjustified -- but anyway, I digress.) >> >> This seems to come from commit 868270f23d8d ("acpi-build: tweak acpi >> migration limits", 2014-07-29) and commit 451b157041d2 ("acpi: Align the >> size to 128k", 2020-12-08). >> >> (2) Why is the logic added in those commits insufficient? > > We are dealing with different blobs here (tables_blob vs. cmd_blob). > >> >> What is the exact call tree that triggers the above error? > > [...] > > acpi_build_update()->acpi_build_update()->memory_region_ram_resize()->qemu_ram_resize() > > A longer calltrace can be found in https://bugzilla.redhat.com/show_bug.cgi?id=1927159. > >>>> +++ b/hw/i386/acpi-microvm.c >>>> @@ -255,7 +255,8 @@ void acpi_setup_microvm(MicrovmMachineState *mms) >>>> ACPI_BUILD_TABLE_MAX_SIZE); >>>> acpi_add_rom_blob(acpi_build_no_update, NULL, >>>> tables.linker->cmd_blob, >>>> - "etc/table-loader", 0); >>>> + ACPI_BUILD_LOADER_FILE, >>>> + ACPI_BUILD_LOADER_MAX_SIZE); >>>> acpi_add_rom_blob(acpi_build_no_update, NULL, >>>> tables.rsdp, >>>> ACPI_BUILD_RSDP_FILE, 0); >> >> (3) Why are we using a different "tool" here, from the previous >> approach? We're no longer setting the GArray sizes; instead, we make the >> "rom->romsize" fields diverge from -- put differently, grow beyond -- >> "rom->datasize". Why is that useful? What are the consequences? > > See ACPI_BUILD_TABLE_MAX_SIZE handling just in the acpi_add_rom_blob() above. > >> >> Where is it ensured that data between "rom->datasize" and "rom->romsize" >> reads as zeroes? > We only expose the current memory_region_size() to our guest, which is > always multiples of 4k pages. > > rom->datasize and rom->romsize will be multiple of 4k AFAIKs. > > acpi_align_size()-> g_array_set_size() will take care of zeroing out > any unused parts within a single 4k page. > > So all unused, guest-visible part should always be 0 I think. > >> >> >>>> diff --git a/include/hw/acpi/aml-build.h b/include/hw/acpi/aml-build.h >>>> index 380d3e3924..93cdfd4006 100644 >>>> --- a/include/hw/acpi/aml-build.h >>>> +++ b/include/hw/acpi/aml-build.h >>>> @@ -6,6 +6,7 @@ >>>> >>>> /* Reserve RAM space for tables: add another order of magnitude. */ >>>> #define ACPI_BUILD_TABLE_MAX_SIZE 0x200000 >>>> +#define ACPI_BUILD_LOADER_MAX_SIZE 0x40000 >>>> >>>> #define ACPI_BUILD_APPNAME6 "BOCHS " >>>> #define ACPI_BUILD_APPNAME8 "BXPC " >>> >> >> The commit message says "Let's increase the maximum size from 4k to >> 64k", and I have two problems with that: >> >> (4a) I have no idea where the current "4k" size comes from. (In case the >> 4k refers to ACPI_BUILD_ALIGN_SIZE, then why are we not changing that >> macro?) > > Changing ACPI_BUILD_ALIGN_SIZE would affect the legacy_table_size in > acpi_build() - so that can't be right. > > What would also work is something like (to be improved) > > diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c > index 45ad2f9533..49cfedddc8 100644 > --- a/hw/i386/acpi-build.c > +++ b/hw/i386/acpi-build.c > @@ -81,6 +81,8 @@ > #define ACPI_BUILD_LEGACY_CPU_AML_SIZE 97 > #define ACPI_BUILD_ALIGN_SIZE 0x1000 > > +#define ACPI_BUILD_LOADER_ALIGN_SIZE 0x2000 > + > #define ACPI_BUILD_TABLE_SIZE 0x20000 > > /* #define DEBUG_ACPI_BUILD */ > @@ -2613,10 +2615,10 @@ void acpi_build(AcpiBuildTables *tables, MachineState *machine) > error_printf("Try removing CPUs, NUMA nodes, memory slots" > " or PCI bridges."); > } > - acpi_align_size(tables_blob, ACPI_BUILD_TABLE_SIZE); > + acpi_align_size(tables_blob, ACPI_BUILD_ALIGN_SIZE); > } > > - acpi_align_size(tables->linker->cmd_blob, ACPI_BUILD_ALIGN_SIZE); > + acpi_align_size(tables->linker->cmd_blob, ACPI_BUILD_LOADER_ALIGN_SIZE); > > > At least for hw/i386/acpi-build.c. > > We will end up creating the resizeable memory region/RAMBlock always with > a size=maximum_size=8k. (could also go for 64k here) > > The only downside is that we might expose a bigger area to the > guest than necessary (e.g., 8k instead of 4k) and will e.g., migrate > 8k instead of 4k (not that we care). > > > On incoming migration from older QEMU versions, we should be able to just > shrink back from 8k to 4k - so migration from older QEMY versions should > continue working just fine. Correction: Older QEMU versions (e.g., before 62be4e3a5041e84304aa23637da623a205c53ecc) did not support resizeable RAM MemoryRegions / RAMBlocks. This affects ~ < QEMU v2.3.0. So unconditionally changing the size of the cmd_blob memory region (e.g., 4k -> 8k) would most probably break migration from never QEMU to older QEMU (v2.2.0.). Not sure if we really care. @MST, Igor what's your take? -- Thanks, David / dhildenb