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Re: [PATCH for-5.0 v2 2/3] fw_cfg: Migrate ACPI table mr sizes separatel

From: Philippe Mathieu-Daudé
Subject: Re: [PATCH for-5.0 v2 2/3] fw_cfg: Migrate ACPI table mr sizes separately
Date: Tue, 7 Apr 2020 19:03:30 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 4/7/20 4:54 PM, David Hildenbrand wrote:
On 07.04.20 16:34, Michael S. Tsirkin wrote:
On Tue, Apr 07, 2020 at 04:17:46PM +0200, Philippe Mathieu-Daudé wrote:
On 4/3/20 12:18 PM, Shameer Kolothum wrote:
Any sub-page size update to ACPI MRs will be lost during
migration, as we use aligned size in ram_load_precopy() ->
qemu_ram_resize() path. This will result in inconsistency in
FWCfgEntry sizes between source and destination. In order to avoid
this, save and restore them separately during migration.

Up until now, this problem may not be that relevant for x86 as both
ACPI table and Linker MRs gets padded and aligned. Also at present,
qemu_ram_resize() doesn't invoke callback to update FWCfgEntry for
unaligned size changes. But since we are going to fix the
qemu_ram_resize() in the subsequent patch, the issue may become
more serious especially for RSDP MR case.

Moreover, the issue will soon become prominent in arm/virt as well
where the MRs are not padded or aligned at all and eventually have
acpi table changes as part of future additions like NVDIMM hot-add

Suggested-by: David Hildenbrand <address@hidden>
Signed-off-by: Shameer Kolothum <address@hidden>
Acked-by: David Hildenbrand <address@hidden>
v1 --> v2
   - Changed *_mr_size from size_t to uint64_t to address portability.
   - post_copy only done if sizes are not aligned.

Please find previous discussions here,
   hw/core/machine.c         |  1 +
   hw/nvram/fw_cfg.c         | 91 ++++++++++++++++++++++++++++++++++++++-
   include/hw/nvram/fw_cfg.h |  6 +++
   3 files changed, 97 insertions(+), 1 deletion(-)

diff --git a/hw/core/machine.c b/hw/core/machine.c
index de0c425605..c1a444cb75 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -39,6 +39,7 @@ GlobalProperty hw_compat_4_2[] = {
       { "usb-redir", "suppress-remote-wake", "off" },
       { "qxl", "revision", "4" },
       { "qxl-vga", "revision", "4" },
+    { "fw_cfg", "acpi-mr-restore", "false" },
   const size_t hw_compat_4_2_len = G_N_ELEMENTS(hw_compat_4_2);
diff --git a/hw/nvram/fw_cfg.c b/hw/nvram/fw_cfg.c
index 179b302f01..4be6c9d9fd 100644
--- a/hw/nvram/fw_cfg.c
+++ b/hw/nvram/fw_cfg.c
@@ -39,6 +39,7 @@
   #include "qemu/config-file.h"
   #include "qemu/cutils.h"
   #include "qapi/error.h"
+#include "hw/acpi/aml-build.h"
   #define FW_CFG_FILE_SLOTS_DFLT 0x20
@@ -610,6 +611,55 @@ bool fw_cfg_dma_enabled(void *opaque)
       return s->dma_enabled;
+static bool fw_cfg_acpi_mr_restore(void *opaque)
+    FWCfgState *s = opaque;
+    bool mr_aligned;
+    mr_aligned = QEMU_IS_ALIGNED(s->table_mr_size, qemu_real_host_page_size) &&
+                 QEMU_IS_ALIGNED(s->linker_mr_size, qemu_real_host_page_size) 
+                 QEMU_IS_ALIGNED(s->rsdp_mr_size, qemu_real_host_page_size);
+    return s->acpi_mr_restore && !mr_aligned;

This code is hard to review.

Is this equivalent?

     if (!s->acpi_mr_restore) {
         return false;
     if (!QEMU_IS_ALIGNED(s->table_mr_size, qemu_real_host_page_size)) {
         return false;
     if (!QEMU_IS_ALIGNED(s->linker_mr_size, qemu_real_host_page_size)) {
         return false;
     if (!QEMU_IS_ALIGNED(s->rsdp_mr_size, qemu_real_host_page_size)) {
         return false;
     return true;

I think I prefer the original version though. Matter of taste?

At least I find the original code fairly easy to read - just as the
proposed alternative. So, yes, matter of taste I'd say.

OK :)

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