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

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Date:   Tue, 2 Feb 2021 10:55:36 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     ira.weiny@...el.com, Jarkko Sakkinen <jarkko@...nel.org>
Cc:     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 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!