Re: [Qemu-devel] [PATCH for-4.1] q35: acpi: do not create dummy MCFG table

[Wei Yang]

On Tue, Apr 09, 2019 at 05:00:37PM +0200, Igor Mammedov wrote:
>Dummy table (with signature "QEMU") creation came from original SeaBIOS
>codebase. And QEMU would have to keep it around if there were Q35 machine
>that depended on keeping ACPI tables blob constant size. Luckily there
>were no versioned Q35 machine types before commit:
>  (since 2.3) a1666142db acpi-build: make ROMs RAM blocks resizeable
>which obsoleted need to keep ACPI tables blob the same size on 
>source/destination.
>
>Considering the 1st versioned machine is pc-q35-2.4, the dummy table
>is not really necessary and it's safe to drop it without breaking
>cross version migration in both directions unconditionally.
>
>Signed-off-by: Igor Mammedov <address@hidden>

Reviewed-by: Wei Yang <address@hidden>

>---
> hw/i386/acpi-build.c | 18 ++++--------------
> 1 file changed, 4 insertions(+), 14 deletions(-)
>
>diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
>index 416da31..8671e25 100644
>--- a/hw/i386/acpi-build.c
>+++ b/hw/i386/acpi-build.c
>@@ -2401,7 +2401,6 @@ static void
> build_mcfg_q35(GArray *table_data, BIOSLinker *linker, AcpiMcfgInfo *info)
> {
>     AcpiTableMcfg *mcfg;
>-    const char *sig;
>     int len = sizeof(*mcfg) + 1 * sizeof(mcfg->allocation[0]);
> 
>     mcfg = acpi_data_push(table_data, len);
>@@ -2411,19 +2410,7 @@ build_mcfg_q35(GArray *table_data, BIOSLinker *linker, 
>AcpiMcfgInfo *info)
>     mcfg->allocation[0].start_bus_number = 0;
>     mcfg->allocation[0].end_bus_number = PCIE_MMCFG_BUS(info->mcfg_size - 1);
> 
>-    /* MCFG is used for ECAM which can be enabled or disabled by guest.
>-     * To avoid table size changes (which create migration issues),
>-     * always create the table even if there are no allocations,
>-     * but set the signature to a reserved value in this case.
>-     * ACPI spec requires OSPMs to ignore such tables.
>-     */
>-    if (info->mcfg_base == PCIE_BASE_ADDR_UNMAPPED) {
>-        /* Reserved signature: ignored by OSPM */
>-        sig = "QEMU";
>-    } else {
>-        sig = "MCFG";
>-    }
>-    build_header(linker, table_data, (void *)mcfg, sig, len, 1, NULL, NULL);
>+    build_header(linker, table_data, (void *)mcfg, "MCFG", len, 1, NULL, 
>NULL);
> }
> 
> /*
>@@ -2592,6 +2579,9 @@ static bool acpi_get_mcfg(AcpiMcfgInfo *mcfg)
>     }
>     mcfg->mcfg_base = qnum_get_uint(qobject_to(QNum, o));
>     qobject_unref(o);
>+    if (mcfg->mcfg_base == PCIE_BASE_ADDR_UNMAPPED) {
>+        return false;
>+    }
> 
>     o = object_property_get_qobject(pci_host, PCIE_HOST_MCFG_SIZE, NULL);
>     assert(o);
>-- 
>2.7.4
>

-- 
Wei Yang
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