linux-kernel - Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit

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Date:   Tue, 11 Jan 2022 10:33:26 -0600
From:   "Haitao Huang" <haitao.huang@...ux.intel.com>
To:     linux-sgx@...r.kernel.org, "Jarkko Sakkinen" <jarkko@...nel.org>,
        "Dave Hansen" <dave.hansen@...ux.intel.com>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Ingo Molnar" <mingo@...hat.com>, "Borislav Petkov" <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        "Kristen Carlson Accardi" <kristen@...ux.intel.com>,
        "Dave Hansen" <dave.hansen@...el.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] x86/sgx: Add accounting for tracking overcommit

On Tue, 11 Jan 2022 09:43:35 -0600, Dave Hansen <dave.hansen@...el.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.

> This isn't about a "heavy enclave load".  If there is *that* much  
> swapped-out enclave memory, will an enclave even make meaningful forward  
> progress?