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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 03.03.21 17:09, Igor Mammedov wrote: > On Wed, 3 Mar 2021 16:03:36 +0100 > Laszlo Ersek wrote: > >> On 03/02/21 19:43, David Hildenbrand wrote: >> >>> We are dealing with different blobs here (tables_blob vs. cmd_blob). >> >> OK, thanks -- this was the important bit I was missing. Over time I've >> lost track of the actual set of fw_cfg blobs that QEMU exposes, for the >> purposes of the ACPI linker/loader. >> >> I've looked up the acpi_add_rom_blob() calls in "hw/i386/acpi-build.c" >> and "hw/arm/virt-acpi-build.c": >> >> hw name max_size notes >> ------- ------------------------------------------- ------------------------------------ ------ >> >> virt ACPI_BUILD_TABLE_FILE ("etc/acpi/tables") ACPI_BUILD_TABLE_MAX_SIZE (0x200000) n/a >> virt ACPI_BUILD_LOADER_FILE ("etc/table-loader") 0 n/a >> virt ACPI_BUILD_RSDP_FILE ("etc/acpi/rsdp") 0 simply modeled on i386 (below) >> >> i386 ACPI_BUILD_TABLE_FILE ("etc/acpi/tables") ACPI_BUILD_TABLE_MAX_SIZE (0x200000) n/a >> i386 ACPI_BUILD_LOADER_FILE ("etc/table-loader") 0 n/a >> i386 ACPI_BUILD_RSDP_FILE ("etc/acpi/rsdp") 0 d70414a5788c, 358774d780ee8 >> >> microvm ACPI_BUILD_TABLE_FILE ("etc/acpi/tables") ACPI_BUILD_TABLE_MAX_SIZE (0x200000) n/a >> microvm "etc/table-loader" 0 no macro for name??? >> microvm ACPI_BUILD_RSDP_FILE ("etc/acpi/rsdp") 0 simply modeled on i386 (above) >> >> (I notice there are some other (optional) fw_cfg blobs too, related TPM, >> vmgenid, nvdimm etc, using fw_cfg_add_file() rather than >> acpi_add_rom_blob() -- so those are immutable (never regenerated). I >> definitely needed this reminder...) > > most of them are just guest RAM reservations (guest/hose exchange buffer) > and "etc/tpm/config" seems to immutable for specific configuration > > >> So, my observations: >> >> (1) microvm open-codes "etc/table-loader", rather than using the macro >> ACPI_BUILD_LOADER_FILE. >> >> The proposed patch corrects it, which I welcome per se. However, it >> should arguably be a separate patch. I found it distracting, in spite of >> the commit message highlighting it. I don't insist though, I'm >> admittedly rusty on this code. >> >> >> (2) The proposed patch sets "max_size" to ACPI_BUILD_LOADER_MAX_SIZE for >> each ACPI_BUILD_LOADER_FILE. Makes sense, upon constructing / reviewing >> the above table. >> >> (I'm no longer sure if tweaking the alignment were the preferable path >> forward.) >> >> Either way, I'd request including the above table in the commit message. >> (Maybe drop the "notes" column.) >> >> >> (3) The above 9 invocations are *all* of the acpi_add_rom_blob() >> invocations. I find the interface brittle. It's not helpful to have so >> many macros for the names and the max sizes. We should have a table with >> three entries and -- minimally -- two columns, specifying name and >> max_size -- possibly some more call arguments, if such can be extracted. >> We should also have an enum type for selecting a row in this table, and >> then acpi_add_rom_blob() should be called with an enum constant. >> >> Of course, talk is cheap. :) >> >> >> (4) When do we plan to introduce a nonzero "max_size" for >> ACPI_BUILD_RSDP_FILE ("etc/acpi/rsdp")? >> >> Is the current zero value a time bomb? > > it's not likely to go over 4k, but if we enforce max_size!=0 we may set it 4k, > which it's aligned to anyways. Interestingly, the size is not aligned. We end up calling rom_add_blob() with a size like "22". The memory region we create has size=22 / max_size=22. (max_size is not effective, we rely on the one from the underlying RAMBlock). The resizeable RAMBlock, however, aligns both up to full pages (e.g., 4k / 4k), so we can later grow it > 22 bytes. Doesn't really matter in practice I guess, because we always expose full pages to the guest, and migrate full pages. One corner case could be shrinking e.g., from 22 bytes to 14 bytes. I am not sure if we would zero-out these 8 bytes somewhere. Not sure if that is of any relevance. Beautiful code. -- Thanks, David / dhildenb