On Wed, 3 Mar 2021 16:03:36 +0100 Laszlo Ersek <ler...@redhat.com> 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. > Put differently: acpi_add_rom_blob() should be *impossible* to call with > "max_size=0", arguably. *Whenever* we call acpi_add_rom_blob(), we do > that because the blob is resizable (mutable) -- but that also means we > should have a safety margin, does it not? So calling acpi_add_rom_blob() > with "max_size=0" looks self-contradictory. main use-case for using acpi_add_rom_blob() is for mutable blobs, so that all these blobs were transferred during migration to the destination, to ensure that guest sees consistent data set (from source instead of mix of source/dst blobs). Resize came later on, when we got sick of ad-hock (align)/size bumping of "etc/acpi/tables" in configurations where size was on verge of crossing border to the next aligned size and related knobs to keep that mess migratable. > > FWIW, this could be covered by the table proposed in point (3). > > > In total, I don't disagree with the patch (beyond the fact that the new > macro's value doesn't match the commit message), functionally speaking. > However, wrt. readability, I think the patch further complicates the > code. I'd suggest five patches: > > #1 -- use "etc/table-loader" via the proper macro name in "microvm", > > #2 -- rework acpi_add_rom_blob() for using a table of constants + an > enum type, > > #3 -- bump the "max_size" field for ACPI_BUILD_LOADER_FILE, for the > current symptom, > > #4 -- set a nonzero "max_size" for the remaining ACPI_BUILD_RSDP_FILE, > for "future-proofing", > > #5 -- in the new acpi_add_rom_blob() implementation, taking the enum, > assert(max_size != 0). > > (I haven't thought through what this would mean for migration, forward > or backward; I'm just brain-storming.) > > Thanks > Laszlo