EPYC 3rd Gen, Resizable BAR support

ectoplasmosis

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Jul 28, 2021
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Can ‘Resizable BAR’ be enabled on EPYC Milan?

I use EPYC 7443P on ROMED8-2T servers for broadcast video with RTX A6000 GPUs which support resizable BAR, but have not found a way to enable it in the BIOS.

Is this even possible on EPYC?

I have Zen3-based Ryzen systems on X570 boards which can enable this feature.
 

RolloZ170

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Apr 24, 2016
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it is possible on threadripper 2000/3000 series (MSI, ASUS, with BIOS update)
but EPYC is system on chip, not dedicated chipset like TR.
 

ectoplasmosis

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Jul 28, 2021
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it is possible on threadripper 2000/3000 series (MSI, ASUS, with BIOS update)
but EPYC is system on chip, not dedicated chipset like TR.
Apparently Resizable BAR is an option in the BIOS of the Asrock EPYCD8-2T with a Rome CPU...

Any other EPYC boards with this feature in the BIOS?
 

ectoplasmosis

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Jul 28, 2021
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can't find that string in the BIOS. how is that called in the BIOS ?
screenshot please !
@Keith Myers mentioned that his EPYCD8-2T has this option.

I only own ROMED8-2T boards, some on BIOS 1.30 with EPYC Gen2 CPUs, and some on L3.20 with EPYC Gen3.

Resizable BAR support would be very helpful for us to have.
 

mirrormax

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Apr 10, 2020
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Win-Raid Forum (Drivers - Firmware - Modding) would be the best place to ask, in theory it should be doable with a modified bios, i dont assume any of the MB manufacturers will bother supporting it on server boards if its mostly a gaming thing but who knows.
reading up on it seems like the only prerequisites is above 4g encoding support and a bios to enable it.

Code is required on Mobo, GPU ROM, Driver and OS. All 4 have to support it in their own code. Windows has supported since 2017.

A quick way to take a top down view is:
1. OS allows this communication to applications,
2. GPU driver allows this communication between OS and GPU,
3. and GPU ROM allows communication to Mobo to adjust the transfer unit
4. Mobo code adjusts its PCIe transfer unit settings

This is how I understood it works. It is a chain of command, and everyone in the chain has to have the code to support it.
 
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RolloZ170

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The implementations are based on a PCIe extension, which has been common in the professional environment for many years:
Resizable BAR support improves communication between CPU and GPU,
by blunting larger as well as variable amounts of data in parallel rather than sequentially
written in 256-Mbyte blocks into the memory of the graphics card.

In the mainboard firmware it is still important: activate the Above 4G decoding and the Resizable BAR support in the PCIe setting;
In addition, the CSM (Compatibility Support Module) at AMD and Nvidia must be deactivated so that Windows runs in UEFI mode.
 
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Keith Myers

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Oct 10, 2020
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I would simply request that feature from Asrock. Tell them the feature is broken or missing from the BIOS for your needs.

When I reported that the System Inventory was broken in the official released BIOS, they were quick to cobble up the fixed BIOS and BMC firmware that provides that feature.
 
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j.battermann

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Aug 22, 2016
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I've reached out to William of ASRock USA, he checked back with their bios development team and got this answer:

1667932593528.png

So no Resizable BAR for SP3 :-/
 

zir_blazer

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Dec 5, 2016
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Is nearly impossible than ReBAR is not supported. nVidia Teslas have been using it from before it became a hit on consumer space with AMD SAM, and EPYC has been popular for compute nodes with multiple nVidia GPUs, so...

For reference, a Tesla A100 is supposed to use 64 GB BARs, and here is a Dual EPYC 7742 with 8 Tesla A100, which would most likely not work if big BAR sizes weren't supported. Note that I don't know whenever Teslas rely on ReBAR (Either standard 256 MiB BAR or a bigger BAR size like consumer cards) or if they are only big BAR, in which case, it may not actually be ReBAR.
 
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RolloZ170

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Apr 24, 2016
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Somebody needs to find the setting in the BIOS' then
there ar etwo reasons for hidden options in the BIOS.
1) options only for admins or hardware engineers/debug.
2) the function is not implemented in any module of the BIOS.
 

Usernae

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Nov 15, 2022
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gigabyte mz32-ar0 v1.0(latest bios) + epyc 7351p + Tesla m40 + arc a380 . Can't find any option about resizable bar in bios. The tesla m40 has resizable bar by default , but a380 doesn't. I have tried move a380 to m40 's slot , it doesn't help. And my a380 have a 43 code and doesn't work with any driver later than 31.0.101.3277.I don't know if it's because a380 is upset not having the resizable bar.
 

RolloZ170

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Apr 24, 2016
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AFAIK to enable re-size BAR in BIOS you have to disable CSM, and for this its required to BOOT in UEFI mode to use re-sizable BAR.
for all who still BOOT in legacy its time for a new fresh UEFI install. i wish all linux users a "good luck"