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Re: [Qemu-devel] How to reserve guest physical region for ACPI


From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] How to reserve guest physical region for ACPI
Date: Mon, 28 Dec 2015 14:50:15 +0200

On Mon, Dec 28, 2015 at 10:39:04AM +0800, Xiao Guangrong wrote:
> 
> Hi Michael, Paolo,
> 
> Now it is the time to return to the challenge that how to reserve guest
> physical region internally used by ACPI.
> 
> Igor suggested that:
> | An alternative place to allocate reserve from could be high memory.
> | For pc we have "reserved-memory-end" which currently makes sure
> | that hotpluggable memory range isn't used by firmware
> (https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg00926.html)

I don't want to tie things to reserved-memory-end because this
does not scale: next time we need to reserve memory,
we'll need to find yet another way to figure out what is where.

I would like ./hw/acpi/bios-linker-loader.c interface to be extended to
support 64 bit RAM instead (and maybe a way to allocate and
zero-initialize buffer without loading it through fwcfg), this way bios
does the allocation, and addresses can be patched into acpi.

See patch at the bottom that might be handy.

> he also innovated a way to use 64-bit address in DSDT/SSDT.rev = 1:
> | when writing ASL one shall make sure that only XP supported
> | features are in global scope, which is evaluated when tables
> | are loaded and features of rev2 and higher are inside methods.
> | That way XP doesn't crash as far as it doesn't evaluate unsupported
> | opcode and one can guard those opcodes checking _REV object if neccesary.
> (https://lists.nongnu.org/archive/html/qemu-devel/2015-11/msg01010.html)

Yes, this technique works.

An alternative is to add an XSDT, XP ignores that.
XSDT at the moment breaks OVMF (because it loads both
the RSDT and the XSDT, which is wrong), but I think
Laszlo was working on a fix for that.

> Michael, Paolo, what do you think about these ideas?
> 
> Thanks!



So using a patch below, we can add Name(PQRS, 0x0) at the top of the
SSDT (or bottom, or add a separate SSDT just for that).  It returns the
current offset so we can add that to the linker.

Won't work if you append the Name to the Aml structure (these can be
nested to arbitrary depth using aml_append), so using plain GArray for
this API makes sense to me.

--->

acpi: add build_append_named_dword, returning an offset in buffer

This is a very limited form of support for runtime patching -
similar in functionality to what we can do with ACPI_EXTRACT
macros in python, but implemented in C.

This is to allow ACPI code direct access to data tables -
which is exactly what DataTableRegion is there for, except
no known windows release so far implements DataTableRegion.

Signed-off-by: Michael S. Tsirkin <address@hidden>

---

diff --git a/include/hw/acpi/aml-build.h b/include/hw/acpi/aml-build.h
index 1b632dc..f8998ea 100644
--- a/include/hw/acpi/aml-build.h
+++ b/include/hw/acpi/aml-build.h
@@ -286,4 +286,7 @@ void acpi_build_tables_cleanup(AcpiBuildTables *tables, 
bool mfre);
 void
 build_rsdt(GArray *table_data, GArray *linker, GArray *table_offsets);
 
+int
+build_append_named_dword(GArray *array, const char *name_format, ...);
+
 #endif
diff --git a/hw/acpi/aml-build.c b/hw/acpi/aml-build.c
index 0d4b324..7f9fa65 100644
--- a/hw/acpi/aml-build.c
+++ b/hw/acpi/aml-build.c
@@ -262,6 +262,32 @@ static void build_append_int(GArray *table, uint64_t value)
     }
 }
 
+/* Build NAME(XXXX, 0x0) where 0x0 is encoded as a qword,
+ * and return the offset to 0x0 for runtime patching.
+ *
+ * Warning: runtime patching is best avoided. Only use this as
+ * a replacement for DataTableRegion (for guests that don't
+ * support it).
+ */
+int
+build_append_named_qword(GArray *array, const char *name_format, ...)
+{
+    int offset;
+    va_list ap;
+
+    va_start(ap, name_format);
+    build_append_namestringv(array, name_format, ap);
+    va_end(ap);
+
+    build_append_byte(array, 0x0E); /* QWordPrefix */
+
+    offset = array->len;
+    build_append_int_noprefix(array, 0x0, 8);
+    assert(array->len == offset + 8);
+
+    return offset;
+}
+
 static GPtrArray *alloc_list;
 
 static Aml *aml_alloc(void)





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