[PATCH] efi_loader: avoid superfluous variable store writes on unchanged data
Michal Simek
michal.simek at amd.com
Thu Mar 12 14:27:20 CET 2026
Every SetVariable() call triggers efi_var_mem_ins() followed by
efi_var_to_storage(), even when the variable value is not actually
changing. This is unfriendly to flash-backed stores that suffer
wear from unnecessary erase/write cycles.
Add a change-detection path to efi_var_mem_ins(): when size2 == 0
(i.e. not an append) and the caller passes a non-NULL changep flag,
look up the existing variable and compare attributes, length, time
and data byte-by-byte. If everything matches, set *changep = false
and return EFI_SUCCESS without touching the variable buffer.
Both efi_set_variable_int() and efi_set_variable_runtime() now
check the flag and skip efi_var_mem_del() / efi_var_to_storage()
when nothing changed.
Signed-off-by: Michal Simek <michal.simek at amd.com>
---
include/efi_variable.h | 8 +++++++-
lib/efi_loader/efi_var_common.c | 2 +-
lib/efi_loader/efi_var_mem.c | 29 ++++++++++++++++++++++++++++-
lib/efi_loader/efi_variable.c | 27 ++++++++++++++++++---------
4 files changed, 54 insertions(+), 12 deletions(-)
diff --git a/include/efi_variable.h b/include/efi_variable.h
index fc1184e5ca1a..c3229c717d89 100644
--- a/include/efi_variable.h
+++ b/include/efi_variable.h
@@ -216,6 +216,11 @@ void efi_var_mem_del(struct efi_var_entry *var);
* The variable is appended without checking if a variable of the same name
* already exists. The two data buffers are concatenated.
*
+ * When @changep is non-NULL and @size2 is 0, the function compares the new
+ * value against an existing variable with the same name and vendor. If
+ * attributes and data are identical the insertion is skipped and *@changep
+ * is set to false, avoiding superfluous writes.
+ *
* @variable_name: variable name
* @vendor: GUID
* @attributes: variable attributes
@@ -224,13 +229,14 @@ void efi_var_mem_del(struct efi_var_entry *var);
* @size2: size of the second data field
* @data2: second data buffer
* @time: time of authentication (as seconds since start of epoch)
+ * @changep: pointer to change flag (may be NULL)
* Result: status code
*/
efi_status_t efi_var_mem_ins(const u16 *variable_name,
const efi_guid_t *vendor, u32 attributes,
const efi_uintn_t size1, const void *data1,
const efi_uintn_t size2, const void *data2,
- const u64 time);
+ const u64 time, bool *changep);
/**
* efi_var_mem_free() - determine free memory for variables
diff --git a/lib/efi_loader/efi_var_common.c b/lib/efi_loader/efi_var_common.c
index c89a4fce4ff8..d63c2d1b1cd8 100644
--- a/lib/efi_loader/efi_var_common.c
+++ b/lib/efi_loader/efi_var_common.c
@@ -526,7 +526,7 @@ efi_status_t efi_var_restore(struct efi_var_file *buf, bool safe)
continue;
ret = efi_var_mem_ins(var->name, &var->guid, var->attr,
var->length, data, 0, NULL,
- var->time);
+ var->time, NULL);
if (ret != EFI_SUCCESS)
log_err("Failed to set EFI variable %ls\n", var->name);
}
diff --git a/lib/efi_loader/efi_var_mem.c b/lib/efi_loader/efi_var_mem.c
index 31180df9e3a0..e364672db81a 100644
--- a/lib/efi_loader/efi_var_mem.c
+++ b/lib/efi_loader/efi_var_mem.c
@@ -159,12 +159,39 @@ efi_status_t __efi_runtime efi_var_mem_ins(
const efi_guid_t *vendor, u32 attributes,
const efi_uintn_t size1, const void *data1,
const efi_uintn_t size2, const void *data2,
- const u64 time)
+ const u64 time, bool *changep)
{
u16 *data;
struct efi_var_entry *var;
u32 var_name_len;
+ if (changep)
+ *changep = true;
+
+ /*
+ * If this is not an append (size2 == 0), check whether the variable
+ * already exists with identical attributes and data. When nothing
+ * changed we can skip the write and avoid superfluous erases.
+ */
+ if (!size2 && changep) {
+ struct efi_var_entry *old;
+
+ old = efi_var_mem_find(vendor, variable_name, NULL);
+ if (old && old->attr == attributes &&
+ old->length == size1 && old->time == time) {
+ u16 *old_data;
+
+ for (old_data = old->name; *old_data; ++old_data)
+ ;
+ ++old_data;
+
+ if (!memcmp(old_data, data1, size1)) {
+ *changep = false;
+ return EFI_SUCCESS;
+ }
+ }
+ }
+
var = (struct efi_var_entry *)
((uintptr_t)efi_var_buf + efi_var_buf->length);
var_name_len = u16_strlen(variable_name) + 1;
diff --git a/lib/efi_loader/efi_variable.c b/lib/efi_loader/efi_variable.c
index 8512bc20f11e..9923936c1b59 100644
--- a/lib/efi_loader/efi_variable.c
+++ b/lib/efi_loader/efi_variable.c
@@ -277,6 +277,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
struct efi_var_entry *var;
efi_uintn_t ret;
bool append, delete;
+ bool changed = false;
u64 time = 0;
enum efi_auth_var_type var_type;
@@ -366,6 +367,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
if (delete) {
/* EFI_NOT_FOUND has been handled before */
attributes = var->attr;
+ changed = true;
ret = EFI_SUCCESS;
} else if (append && var) {
/*
@@ -380,15 +382,19 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
ret = efi_var_mem_ins(variable_name, vendor,
attributes & ~EFI_VARIABLE_APPEND_WRITE,
var->length, old_data, data_size, data,
- time);
+ time, &changed);
} else {
ret = efi_var_mem_ins(variable_name, vendor, attributes,
- data_size, data, 0, NULL, time);
+ data_size, data, 0, NULL, time,
+ &changed);
}
if (ret != EFI_SUCCESS)
return ret;
+ if (!changed)
+ return EFI_SUCCESS;
+
efi_var_mem_del(var);
if (var_type == EFI_AUTH_VAR_PK)
@@ -396,10 +402,7 @@ efi_status_t efi_set_variable_int(const u16 *variable_name,
else
ret = EFI_SUCCESS;
- /*
- * Write non-volatile EFI variables
- * TODO: check if a value change has occured to avoid superfluous writes
- */
+ /* Write non-volatile EFI variables to storage */
if (attributes & EFI_VARIABLE_NON_VOLATILE) {
if (IS_ENABLED(CONFIG_EFI_VARIABLE_NO_STORE))
return EFI_SUCCESS;
@@ -498,6 +501,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
struct efi_var_entry *var;
efi_uintn_t ret;
bool append, delete;
+ bool changed = false;
u64 time = 0;
if (!IS_ENABLED(CONFIG_EFI_RT_VOLATILE_STORE))
@@ -549,6 +553,7 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
if (delete) {
/* EFI_NOT_FOUND has been handled before */
attributes = var->attr;
+ changed = true;
ret = EFI_SUCCESS;
} else if (append && var) {
u16 *old_data = (void *)((uintptr_t)var->name +
@@ -556,15 +561,19 @@ efi_set_variable_runtime(u16 *variable_name, const efi_guid_t *vendor,
ret = efi_var_mem_ins(variable_name, vendor, attributes,
var->length, old_data, data_size, data,
- time);
+ time, &changed);
} else {
ret = efi_var_mem_ins(variable_name, vendor, attributes,
- data_size, data, 0, NULL, time);
+ data_size, data, 0, NULL, time,
+ &changed);
}
if (ret != EFI_SUCCESS)
return ret;
- /* We are always inserting new variables, get rid of the old copy */
+
+ if (!changed)
+ return EFI_SUCCESS;
+
efi_var_mem_del(var);
return EFI_SUCCESS;
--
2.43.0
base-commit: 1e8eb1c27210e3beb5d3f4402a40d2e8886bcf9f
branch: debian-sent3
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