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Subject: Re: [PATCH v11 09/13] x86, sgx: basic routines for enclave page cache
To:     Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>, x86@kernel.org,
        platform-driver-x86@vger.kernel.org
Cc:     dave.hansen@intel.com, sean.j.christopherson@intel.com,
        nhorman@redhat.com, npmccallum@redhat.com,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>,
        "open list:INTEL SGX" <intel-sgx-kernel-dev@lists.01.org>
References: <20180608171216.26521-1-jarkko.sakkinen@linux.intel.com>
 <20180608171216.26521-10-jarkko.sakkinen@linux.intel.com>
From:   Jethro Beekman <jethro@fortanix.com>
Message-ID: <78982a32-c589-48e2-9a83-fd36903b5588@fortanix.com>
Date:   Fri, 8 Jun 2018 11:21:48 -0700
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On 2018-06-08 10:09, Jarkko Sakkinen wrote:
> +/*
> + * Writing the LE hash MSRs is extraordinarily expensive, e.g.
> + * 3-4x slower than normal MSRs, so we use a per-cpu cache to
> + * track the last known value of the MSRs to avoid unnecessarily
> + * writing the MSRs with the current value.  Because most Linux
> + * kernels will use an LE that is signed with a non-Intel key,

I don't think you can predict what most Linux kernels will be doing. I=20
think not initializing the cache to the CPU's initial value is fine, but =

this particular argument shouldn't appear in the rationale.

> + * i.e. the first EINIT will need to write the MSRs regardless
> + * of the cache, the cache is intentionally left uninitialized
> + * during boot as initializing the cache would be pure overhead
> + * for the majority of systems.  Furthermore, the MSRs are per-cpu
> + * and the boot-time values aren't guaranteed to be identical
> + * across cpus, so we'd have to run code all all cpus to properly
> + * init the cache.  All in all, the complexity and overhead of
> + * initializing the cache is not justified.
> + */
> +static DEFINE_PER_CPU(u64 [4], sgx_le_pubkey_hash_cache);

--=20
Jethro Beekman | Fortanix


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