Re: [PATCH] UefiCpuPkg/MtrrLib: Add flag to skip disabling MTRRs prior to MTRR change.

["Ni, Ruiyu" <ruiyu.ni@Intel.com>]

On 9/18/2018 10:57 PM, Duran, Leo wrote:
> 
> 
>> -----Original Message-----
>> From: Ni, Ruiyu [mailto:ruiyu.ni@Intel.com]
>> Sent: Tuesday, September 18, 2018 3:34 AM
>> To: Laszlo Ersek <lersek@redhat.com>; Duran, Leo <leo.duran@amd.com>;
>> edk2-devel@lists.01.org
>> Cc: Dong, Eric <eric.dong@intel.com>
>> Subject: Re: [edk2] [PATCH] UefiCpuPkg/MtrrLib: Add flag to skip disabling
>> MTRRs prior to MTRR change.
>>
>> On 9/18/2018 12:38 AM, Laszlo Ersek wrote:
>>> On 09/17/18 18:20, Duran, Leo wrote:
>>>>
>>>>> -----Original Message-----
>>>>> From: Ni, Ruiyu <ruiyu.ni@Intel.com>
>>>>> Sent: Thursday, September 13, 2018 11:44 PM
>>>>> To: Duran, Leo <leo.duran@amd.com>; Laszlo Ersek
>>>>> <lersek@redhat.com>; edk2-devel@lists.01.org
>>>>> Cc: Dong, Eric <eric.dong@intel.com>
>>>>> Subject: Re: [edk2] [PATCH] UefiCpuPkg/MtrrLib: Add flag to skip
>>>>> disabling MTRRs prior to MTRR change.
>>>>>
>>>>> On 9/14/2018 3:31 AM, Duran, Leo wrote:
>>>>>>
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Ni, Ruiyu <ruiyu.ni@intel.com>
>>>>>>> Sent: Wednesday, September 12, 2018 9:39 PM
>>>>>>> To: Duran, Leo <leo.duran@amd.com>; Laszlo Ersek
>>>>> <lersek@redhat.com>;
>>>>>>> edk2-devel@lists.01.org
>>>>>>> Cc: Dong, Eric <eric.dong@intel.com>
>>>>>>> Subject: RE: [PATCH] UefiCpuPkg/MtrrLib: Add flag to skip
>>>>>>> disabling MTRRs prior to MTRR change.
>>>>>>>
>>>>>>> Leo,
>>>>>>> Sorry I was in leave yesterday so didn't see the mail.
>>>>>>> Which MSRs are shared? Only the
>>>>> MSR_IA32_MTRR_DEF_TYPE_REGISTER?
>>>>>>> Or all the MSRs that configures the CPU MTRR setting?
>>>>>>>
>>>>>>
>>>>>> Hi Ray,
>>>>>> The MTTR config MSRs are also shared by threads within a core.
>>>>>>
>>>>>
>>>>> Hi Leo,
>>>>> Do you think that fixing the caller is more proper?
>>>>
>>>> Hi Ray,
>>>> Actually,
>>>> The proposed PCD is the simplest solution, as that works for us and does
>> not change the existing (default) flow.
>>>>
>>>> That is,
>>>> I'd prefer making a decision about the PCD in platform-specific code,
>> rather than introducing complex detection and heuristics at the caller level in
>> EDK2 (just for AMD).
>>>>
>>>> So, please approve the PCD.
>>
>> Leo,
>> I agree with you on the first part "the PCD is the simplest solution".
>> But this really looks like a workaround of the real issue.
>> For a multiple-socket system, it may contain S sockets, each socket contains C
>> cores and each core contains T threads. In summary the system contains S *
>> C * T threads.
>> As you said all threads inside a core share the MTRR setting.
>> Do all cores inside a socket share the MTRR setting?
>> Do all sockets share the MTRR setting?
>>
>> If one of the answer of above questions is "no", how can we configure the
>> PCD?
>>
> [Duran, Leo]
> Hi Ray,
> The MTTR settings are share by threads within a core (but each core has its own, etc.)
> The PCD would be set in our platform-specific code (e.g., it can be set at build-time in the .DSC file).
> 
> As I mentioned,
> We don't need (Mtrr.Enable=0) to change MTRR settings, so having the PCD to skip (Mtrr.Enable=0) is reasonable for us.
> 
> Leo.
> 

If the PCD is false, no thread disables the MTRR before programming it.
Is it safe? Per Intel's SDM, it's not.

Maybe it works in AMD's case. But I still suggest we change the caller, 
which is more natural.
At least I'd like to see how potential-ugly the change can be.
We can then discuss how to make the ugly change better looking.

>>>
>>> - From my side, if it works for you, it works for me. (The general
>>> trend has been to avoid adding more PCDs to the "core" package DEC
>>> files, but I'm 100% neutral on that.)
>>>
>>> Laszlo
>>>
>>
>> Laszlo,
>> Thanks for pointing out the general trend. Yes less PCDs are very welcomed.
>> To me, PCD is no different from protocol. And even worse, because it's very
>> easily to be over-used.
>> But I am not sure whether a PCD has to be introduced for this issue.
>> Maybe even we choose to fix the caller, the PCD is still needed. I am not
>> sure.
>>
>> --
>> Thanks,
>> Ray
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-- 
Thanks,
Ray