Message-ID: <4996667d-938d-0833-f5a5-bf5ec82f69ea@intel.com>
Date:   Tue, 11 Jan 2022 09:39:30 -0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101
 Thunderbird/91.4.1
Subject: Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit
Content-Language: en-US
To:     Haitao Huang <haitao.huang@linux.intel.com>,
        linux-sgx@vger.kernel.org, Jarkko Sakkinen <jarkko@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
        Kristen Carlson Accardi <kristen@linux.intel.com>
Cc:     linux-kernel@vger.kernel.org
References: <20220107181618.6597-1-kristen@linux.intel.com>
 <20220107181618.6597-2-kristen@linux.intel.com>
 <171631a9-aaec-f590-3377-29c13fc6041e@intel.com>
 <3b169fde457f28e086c49c8a3b8d81ac539cfb59.camel@linux.intel.com>
 <op.1ft4sin7wjvjmi@hhuan26-mobl1.mshome.net>
 <bc370c39-d8c1-9371-2345-cf255ced9a1b@intel.com>
 <op.1fuaql06wjvjmi@hhuan26-mobl1.mshome.net>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <op.1fuaql06wjvjmi@hhuan26-mobl1.mshome.net>
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Precedence: bulk

On 1/11/22 08:33, Haitao Huang wrote:
> On Tue, 11 Jan 2022 09:43:35 -0600, Dave Hansen <dave.hansen@intel.com> 
> wrote:
>> On 1/11/22 06:20, Haitao Huang wrote:
>>> If the system has a ton of RAM but limited EPC, I think it makes 
>>> sense to allow more EPC swapping, can we do min(0.5*RAM, 2*EPC)?
>>> I suppose if the system is used for heavy enclave load, user would be 
>>> willing to at least use half of RAM.
>>
>> If I have 100GB of RAM and 100MB of EPC, can I really *meaningfully* 
>> run 50GB of enclaves?  In that case, if everything was swapped out 
>> evenly, I would only have a 499/500 chance that a given page reference 
>> would fault.
> 
> The formula will cap swapping at 2*EPC so only 200MB swapped out.  So 
> the miss is at most 1/3.
> The original hard coded cap 1.5*EPC may still consume too much RAM if 
> RAM<1.5*EPC.

Oh, sorry, I read that backwards.

Basing it on the amount of RAM is a bit nasty.  You might either really 
overly restrict the amount of allowed EPC, or you have to handle hotplug.