linux-kernel - Re: [PATCH] x86: Remove unnecessary kmap() from sgx_ioc_enclave

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Date:   Wed, 3 Feb 2021 00:43:50 +0200
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     ira.weiny@...el.com, Sean Christopherson <seanjc@...gle.com>,
        Jethro Beekman <jethro@...tanix.com>,
        linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org
Subject: Re: [PATCH] x86: Remove unnecessary kmap() from
 sgx_ioc_enclave_init()

On Tue, Feb 02, 2021 at 10:55:36AM -0800, Dave Hansen wrote:
> On 2/1/21 5:37 PM, ira.weiny@...el.com wrote:
> > kmap is inefficient and we are trying to reduce the usage in the kernel.
> > There is no readily apparent reason why the initp_page page needs to be
> > allocated and kmap'ed() but sigstruct needs to be page aligned and token
> > 512 byte aligned.
> 
> Hi Ira,
> 
> It's a *relatively* recent guaranteed, but:
> 
> https://www.kernel.org/doc/Documentation/core-api/memory-allocation.rst
> 
> says:
> 
> > The address of a chunk allocated with `kmalloc` is aligned to at least
> > ARCH_KMALLOC_MINALIGN bytes.  For sizes which are a power of two, the
> > alignment is also guaranteed to be at least the respective size.
> 
> So, if you allocate a page with kmalloc(), you get an aligned page.  Yay!

And this what we do sgx_ioc_enclave_create() anyway, as I stated in my
earlier response. Better to use the same pattern everywhere consitently
when it makes sense.

/Jarkko