[PATCH v2] x86/sgx: Replace kmap/kunmap_atomic calls

Kristen Carlson Accardi
2022-11-15T16:16:43Z

kmap_local_page() is the preferred way to create temporary mappings
when it is feasible, because the mappings are thread-local and
CPU-local. kmap_local_page() uses per-task maps rather than per-CPU
maps. This in effect removes the need to preemption in the
local CPU while kmap is active, and thus vastly reduces overall
system latency. It is also valid to take pagefaults.

The use of kmap_atomic() in the SGX code was not an explicit design
choice to disable page faults or preemption, and there is no compelling
design reason to using kmap_atomic() vs. kmap_local_page().

Link: https://lore.kernel.org/linux-sgx/Y0biN3%2FJsZMa0yUr@kernel.org/

For more information on the use of kmap_local_page() vs.
kmap_atomic(), please see Documentation/mm/highmem.rst

Signed-off-by: Kristen Carlson Accardi <kristen@linux.intel.com>
---
Changes since V1:

- Reword commit message to make it more clear why it is preferred
to use kmap_local_page() vs. kmap_atomic().

arch/x86/kernel/cpu/sgx/encl.c | 12 ++++++------
arch/x86/kernel/cpu/sgx/ioctl.c | 4 ++--
arch/x86/kernel/cpu/sgx/main.c | 8 ++++----
3 files changed, 12 insertions(+), 12 deletions(-)

diff --git a/arch/x86/kernel/cpu/sgx/encl.c b/arch/x86/kernel/cpu/sgx/encl.c
index 2c258255a629..68f8b18d2278 100644
--- a/arch/x86/kernel/cpu/sgx/encl.c
+++ b/arch/x86/kernel/cpu/sgx/encl.c
@@ -160,8 +160,8 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
return ret;

pginfo.addr = encl_page->desc & PAGE_MASK;
- pginfo.contents = (unsigned long)kmap_atomic(b.contents);
- pcmd_page = kmap_atomic(b.pcmd);
+ pginfo.contents = (unsigned long)kmap_local_page(b.contents);
+ pcmd_page = kmap_local_page(b.pcmd);
pginfo.metadata = (unsigned long)pcmd_page + b.pcmd_offset;

if (secs_page)
@@ -187,8 +187,8 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,
*/
pcmd_page_empty = !memchr_inv(pcmd_page, 0, PAGE_SIZE);

- kunmap_atomic(pcmd_page);
- kunmap_atomic((void *)(unsigned long)pginfo.contents);
+ kunmap_local(pcmd_page);
+ kunmap_local((void *)(unsigned long)pginfo.contents);

get_page(b.pcmd);
sgx_encl_put_backing(&b);
@@ -197,10 +197,10 @@ static int __sgx_encl_eldu(struct sgx_encl_page *encl_page,

if (pcmd_page_empty && !reclaimer_writing_to_pcmd(encl, pcmd_first_page)) {
sgx_encl_truncate_backing_page(encl, PFN_DOWN(page_pcmd_off));
- pcmd_page = kmap_atomic(b.pcmd);
+ pcmd_page = kmap_local_page(b.pcmd);
if (memchr_inv(pcmd_page, 0, PAGE_SIZE))
pr_warn("PCMD page not empty after truncate.\n");
- kunmap_atomic(pcmd_page);
+ kunmap_local(pcmd_page);
}

put_page(b.pcmd);
diff --git a/arch/x86/kernel/cpu/sgx/ioctl.c b/arch/x86/kernel/cpu/sgx/ioctl.c
index ef874828fa6b..03c50f11ad75 100644
--- a/arch/x86/kernel/cpu/sgx/ioctl.c
+++ b/arch/x86/kernel/cpu/sgx/ioctl.c
@@ -221,11 +221,11 @@ static int __sgx_encl_add_page(struct sgx_encl *encl,
pginfo.secs = (unsigned long)sgx_get_epc_virt_addr(encl->secs.epc_page);
pginfo.addr = encl_page->desc & PAGE_MASK;
pginfo.metadata = (unsigned long)secinfo;
- pginfo.contents = (unsigned long)kmap_atomic(src_page);
+ pginfo.contents = (unsigned long)kmap_local_page(src_page);

ret = __eadd(&pginfo, sgx_get_epc_virt_addr(epc_page));

- kunmap_atomic((void *)pginfo.contents);
+ kunmap_local((void *)pginfo.contents);
put_page(src_page);

return ret ? -EIO : 0;
diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c
index 0aad028f04d4..e5a37b6e9aa5 100644
--- a/arch/x86/kernel/cpu/sgx/main.c
+++ b/arch/x86/kernel/cpu/sgx/main.c
@@ -165,17 +165,17 @@ static int __sgx_encl_ewb(struct sgx_epc_page *epc_page, void *va_slot,
pginfo.addr = 0;
pginfo.secs = 0;

- pginfo.contents = (unsigned long)kmap_atomic(backing->contents);
- pginfo.metadata = (unsigned long)kmap_atomic(backing->pcmd) +
+ pginfo.contents = (unsigned long)kmap_local_page(backing->contents);
+ pginfo.metadata = (unsigned long)kmap_local_page(backing->pcmd) +
backing->pcmd_offset;

ret = __ewb(&pginfo, sgx_get_epc_virt_addr(epc_page), va_slot);
set_page_dirty(backing->pcmd);
set_page_dirty(backing->contents);

- kunmap_atomic((void *)(unsigned long)(pginfo.metadata -
+ kunmap_local((void *)(unsigned long)(pginfo.metadata -
backing->pcmd_offset));
- kunmap_atomic((void *)(unsigned long)pginfo.contents);
+ kunmap_local((void *)(unsigned long)pginfo.contents);

return ret;
}
--
2.38.1