HW info reports REBAR supported in Radeon VII…

1 : Anonymous2021/06/15 20:59 ID: o0og9c
HW info reports REBAR supported in Radeon VII...
2 : Anonymous2021/06/16 04:05 ID: h1xjugt

(8) requirements for resizable BAR to function the way you want... to clear up confusion.

- pure UEFI system bios (no CSM)

- GPU vBios reporting to BIOS BAR sizes of VRAM (see PCIE Extended Cfg space Capability structure)

- System BIOS enabling resizable BAR feature (and Above 4G decode, due to 64bit BAR)

- OS (basic driver) recognizing large BAR aperture, that is above 4G, without black screen... eg: Windows 10+ or pretty much any Linux in last 5 years

- Driver detecting feature and correctly reporting to user-mode drivers (DirectX) that it has VRAM size visible memory (instead of 256MB). Also, Vulkan Tools report this "Heap" size.

- User mode drivers (dxx/vulkan/etc/) manage application requests for visible frame buffer, depending on the application and API, they could tune the allocation.

- Game/Application requesting GPU Visible memory type. There are many different kinds of memory: CPU vs GPU. Visible vs Invisible. cached vs uncached. depending on user-mode API.

- Fast CPU and ample PCIE Bandwidth. Older (higher latency) CPUs and platforms older than PCIE 4.0 may not benefit as much, as resizable BAR increases memory transactions over PCIE bus.

Otherwise there is no benefits, even if one tool says "supported" or "enabled"

ID: h1y3gso

Is it the same requirements for intel/nvidia?

ID: h1yswa2

Mostly. The only thing 100% under AMD control is kernel and user mode drivers.

ID: h1y6pph

Yes, as resizable BAR is a standard and not AMD-specific.

ID: h1y7aru

You should add 500 series chipset as requirements because I have all of the above and it does absolutely nothing in games, margin of error results.( Tested with horizon zero dawn, sotTR, cyberpunk and exodus) I think it needs the PCI 4.0 bandwidth to properly work in games or it's an artificial limit set by AMD to make us buy the newer 500 chipset mobos.

ID: h1yrm6m

Great point. Lower then x8 lane width (eg: using chipset ports), maybe even PCIE 3.0 x8, could definitely impact performance gains. Resizable BAR increases the number of memory transactions over the PCIE bus. Nothing is being held back. Its latency sensitive. Ryzen 5xxx improved the latency dramatically over Ryzen 3xxx
One of the many reasons originally only Radeon 6xxx + Ryzen 5xxx CPUs + 5xx series chipsets were supported, was due to the profiling and testing on that configuration. Its disingenuous to claim xx% gains, but then users enable feature on different platform and get +- 1% and feel cheated.

ID: h1ybcf8

Same with my 10900K, ASUS Z490-I and 3070 tuf no difference once so ever.

ID: h1ybrjc

Is that true, even on CPUs without PCIe 4.0? I'm using a R7 1700 on B550 with a 6900 XT. RBAR works.

ID: h1zl8oz


Everything on the PCI 4 bus isn't going to work right when things don't utilize SMT/HYPERTHREADING. I have an nvme drive and it runs like trash.

AMD sponsored games sure as hell use 99% of my CPU though allowing my 6900xt to actually do it's job. It's all bullshit and greed. Unreal engine runs like shit because it uses 10% of my cpu while XBOX TITLES are great. #thumbsfuckingdown

Ps; My rebar support is fully working, b550.

ID: h1zevdy

GPU-Z also has a Resizeable BAR info screen that details each requirement and their current status. Helped me find yesterday that I needed to update my GPU BIOS.

ID: h1y8rwv

well you could release vbios updates to make it supported..just saying..

3 : Anonymous2021/06/15 21:24 ID: h1w9d0k

Rebar can be enabled in the motherboard BIOS. All hwinfo did is poll the mobo BIOS to see if it supports enabling rebar.

To make good use of rebar while gaming requires a vBIOS update.

4 : Anonymous2021/06/16 03:22 ID: h1xfho1

Better off using GPU-Z's information dump to see exactly if and why ReBAR is or isn't working on your system.

ID: h1xk893

I added a top-level comment with the 7 requirements for REBAR. that new page in GPU-Z is very nice, thank you for sharing.

5 : Anonymous2021/06/15 23:00 ID: h1wlc7x

So Polaris has rebar? i got a rx590, already activated rebar on bios, hwinfo shows rebar supported and state enable, thats all? or do i have to wait amd new drivers, vbios?

ID: h1wtevk

From the looks of it, yeah. It's a matter of AMD giving proper driver support for SAM.

ID: h1x9fs0

Ellesmere XT gang checking in. The gift that keeps on giving.

ID: h1xjwlr


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ID: h1xbrx1

Yes but; the polaris is'nt going to benefit alot really compared to a monster like 6900XT. It's there to provide bigger and faster chunks to and back from VRAM. Even if you manage to utilitize the 8GB on a RX580 or so; it's not going to gain you alot since there are not alot of games that would benefit for high resolutions or such. The Polaris itself is more of a 1080p card with some 1440p backing it up; not a by design 4K card.

ID: h1xyrp3

Technically, every PCIe 3.0 GPU has REBAR support(it's standard, default spec), it just needs to be enabled on the driver side.

ID: h1y6ims

Yep, my RX580 has it enabled as well since I updated my motherboard's bios a few months ago.

It surprised me because:

- mobo is X470 series: asus Prime X470-pro (no PCI gen 4)
- cpu is Ryzen 2700X

Net result though is no measurable change in game performance with this combination of hardware

6 : Anonymous2021/06/15 21:25 ID: h1w9elb

Pretty much everything since at least Polaris supports resizable BAR out of the box.

ID: h1wil62

Yes, and it works on Linux, but has AMD actually enabled ReBar on Windows for anything not 6800? Enabling it is one thing, but there is driver optimization.

Also, even if they did, it requires motherboard bios support, which AMD screwed everyone on 370 and nobody bypassed them outside of ASRock, and 470 support is up to the manufacturer to update, and not all of them actually provide those updates.

ID: h1w9wvg

As far as I know it needs a vBIOS update. Am I wrong?

ID: h1wa0it

Only Nvidia cards do.

7 : Anonymous2021/06/16 08:00 ID: h1y25w7

It states this on Vega 64 as well using HWInfo , but...go on GPU-Z and you will see that despite it being supported in BIOS via the motherboard, it will not be supported GPU side. Not to mention there isn't any driver support because the hardware is "unsupported"

8 : Anonymous2021/06/16 00:32 ID: h1wvpxw

Drivers need to be able to enable the compatibility. 🙁

ID: h1x94jq

No, they don’t? You enable it in your motherboards bios.

ID: h1x9bim

In windows the support needs to be enabled in the driver. Simply enabling in the bios does not make Windows use it. It will work in Linux though.

Idiots like you don't read.

9 : Anonymous2021/06/16 02:28 ID: h1x98nr

Yup, I enabled it about 3 months ago on my 5700xt after updating my mobos bios, got a few fps increase in some games.

ID: h1xwv8v

How do you enable it on a 5700 xt? I have the MSI Mech OC and I'm interested, running a Ryzen 7 5800X on an Asus X570 TUF mobo.

ID: h1yvqvk

You might have to update your motherboard bios to get the option. For me it was under pci subsystem settings and was called resizable bar.

10 : Anonymous2021/06/16 00:13 ID: h1wtl56

Isnt rebar just a feature that you have to utilize in a smart way?

Rebar: enabled

Driver: k, I dont know what to even do now. Guess I gonna set it to 8mb for lower layency... wooops, the game requested some thicc textures, guess it's stutter time until tramsfer is complete and bar is set to 1024mb. wooops, now the game requests smol chunks and the bar is too big. Gotta change back to smol. Wooops, big asset is requested again. If only GPU could communicate to me in advance so bar could be set in advance in some smart way that actually benefits the performance...

ID: h1ww684

It just means a far bigger, and configurable, aperture size, instead of just 256MB. This lets the entire VRAM and io regs be directly addressable all at once.

Windows taskmanager reports 16gb address space for my rtx 3060

ID: h1xu1jb

Yeah sure. It might not work as I described, but we've seen performance reduction in some games with SAM or novideo rebar enabled, so there is more to "rebar related performance improvement" than rebar itself.

ID: h1xsps5

Not really how it works. resizeable bar just changes the amount of VRAM the CPU can see directly. It doesn't force the system to load a certain amount of data at a time (such as your suggested 8 MB). An 8 GB BAR size doesn't mean the CPU now has to load 8 GB at a time when it wants to read from VRAM, it just means that it's no longer limited to a maximum of 256MB, and that it it'll always know exactly what's where.

11 : Anonymous2021/06/16 01:05 ID: h1wzlbo

I want SAM for Radeon VII to be out because the bandwidth of HBM2 is totally choked by windows.

12 : Anonymous2021/06/15 21:05 ID: h1w6ujg

How come with no GPU BIOS Update?

ID: h1wecup

Cause AMD made it native on Radeon cards. It wasn't until Windows 10 20H2 that MS added OS support for Resize Bar, that was the major hurdle until now, it went virtually ignored even though it has been been part of the spec since the release of PCIE 3. It had been supported under Linux all this time since the Polaris generation.

ID: h1xkxzc

Great points about Linux and PCIE specification.

Where did you get the idea that 20h2 is a requirement for resizable BAR?

ID: h1wehx8

Good to know. Thanks for the info!

ID: h1wel5h

Because the GPU itself has supported it since launch.... the drivers and mobo BIOS for it weren't ready so they didn't bother pushing the feature out.

ID: h1xazf6

the drivers and mobo BIOS

The drivers were only able to support it on Windows after Microsoft updated the OS. In terms of the BIOS, it's been an optional feature up until now.

13 : Anonymous2021/06/16 00:19 ID: h1wu8rd

my 6800xt h as rebar but there is no noticeable performance difference

14 : Anonymous2021/06/16 07:07 ID: h1xynob

Also works for my Vega64 on a GB Z370 Gaming 7. Supported & Enabled.

ID: h1y3em5

Define "works" please. I'm interested in results.

15 : Anonymous2021/06/16 12:07 ID: h1yjfva

Same on my Vega 64

16 : Anonymous2021/06/16 12:42 ID: h1ymy1f

I haven't updated my bios yet as I am afraid that after that something will not work or work just worse than now

SAM really works only in few games

17 : Anonymous2021/06/16 16:26 ID: h1zg6vy

the bloody thing is just going to die on you anyways. took a day for mine to break


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