Re: [RFC v1] drm/i915: Add Exec param to control data port coherency.

From: Oscar Mateo <oscar.mateo@intel.com>
To: "Lis, Tomasz" <tomasz.lis@intel.com>,
	Chris Wilson <chris@chris-wilson.co.uk>,
	intel-gfx@lists.freedesktop.org
Cc: bartosz.dunajski@intel.com
Subject: Re: [RFC v1] drm/i915: Add Exec param to control data port coherency.
Date: Tue, 20 Mar 2018 11:43:45 -0700	[thread overview]
Message-ID: <ced25d42-a84e-debb-0cae-5a4b5a2d134a@intel.com> (raw)
In-Reply-To: <31312894-68a0-bb5c-04af-348097bff2a1@intel.com>

On 3/19/2018 7:14 AM, Lis, Tomasz wrote:
>
>
> On 2018-03-19 13:43, Chris Wilson wrote:
>> Quoting Tomasz Lis (2018-03-19 12:37:35)
>>> The patch adds a parameter to control the data port coherency 
>>> functionality
>>> on a per-exec call basis. When data port coherency flag value is 
>>> different
>>> than what it was in previous call for the context, a command to 
>>> switch data
>>> port coherency state is added before the buffer to be executed.
>> So this is part of the context? Why do it at exec level?
>
> It is part of the context, stored within HDC chicken bit register.
> The exec level was requested by the OCL team, due to concerns about 
> performance cost of context setparam calls.
>
>>   If exec level
>> is desired, why not whitelist it?
>> -Chris
>
> If we have no issue in whitelisting the register, I'm sure OCL will 
> agree to that.
> I assumed the whitelisting will be unacceptable because of security 
> concerns with some options.
> The register also changes its position and content between gens, which 
> makes whitelisting hard to manage.
>

I think a security analysis of this register was already done, and the 
result was that it contains some other bits that could be dangerous. In 
CNL those bits were moved out of the way and the HDC_CHICKEN0 register 
can be whitelisted (WaAllowUMDToControlCoherency). In ICL the register 
should already be non-privileged.

> Main purpose of chicken bit registers, in general, is to allow work 
> around for hardware features which could  be buggy or could have 
> unintended influence on the platform.
> The data port coherency functionality landed there for the same 
> reasons; then it twisted itself in a way that we now need user space 
> to switch it.
> Is it really ok to whitelist chicken bit registers?
> -Tomasz
>
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