linux-kernel - Re: [PATCH v2] x86/sgx: Replace kmap/kunmap

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Message-ID: <Y3VeFLAYdIDZCRxO@iweiny-mobl>
Date:   Wed, 16 Nov 2022 14:03:00 -0800
From:   Ira Weiny <ira.weiny@...el.com>
To:     Kristen Carlson Accardi <kristen@...ux.intel.com>
CC:     <jarkko@...nel.org>, <dave.hansen@...ux.intel.com>,
        <linux-sgx@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        <x86@...nel.org>, "H. Peter Anvin" <hpa@...or.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] x86/sgx: Replace kmap/kunmap_atomic calls

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@...ux.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@...el.com>

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