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

Ira Weiny
2022-11-16T22:03:13Z

On Tue, Nov 15, 2022 at 08:16:26AM -0800, Kristen Carlson Accardi wrote:
> 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));

NIT: Technically you don't have to pass the pointer to the beginning of the
page here but this works fine.

Reviewed-by: Ira Weiny <ira.weiny@intel.com>

> - kunmap_atomic((void *)(unsigned long)pginfo.contents);
> + kunmap_local((void *)(unsigned long)pginfo.contents);
>
> return ret;
> }
> --
> 2.38.1
>