Asus have finally decided to take action, here are some Z370 Beta BIOS with Resizable BAR support;

Asus Z370 Rog Maximus X Apex BIOS Version 2603 Beta

Asus Z370 Rog Maximus X Hero BIOS Version 2603 Beta

Asus Z370 Rog Maximus X Hero (Wi-Fi AC) BIOS Version 2603 Beta

Asus Z370 Rog Maximus X Formula BIOS Version 2603 Beta

Asus Z370 Rog Maximus X Code BIOS Version 2603 Beta

[Edit] Asus have really stepped up , all Z370 boards appear to now have a Beta BIOS with Resizable BAR support;

Asus Rog Strix Z370-E Gaming BIOS Version 3003 Beta

Asus Rog Strix Z370-F Gaming BIOS Version 3003 Beta

Asus Rog Strix Z370-G Gaming BIOS Version 3003 Beta

Asus Rog Strix Z370-G Gaming (Wi-Fi AC) BIOS Version 3003 Beta

Asus Rog Strix Z370-H Gaming BIOS Version 2603 Beta

Asus Rog Strix Z370-I Gaming BIOS Version 3003 Beta

Asus Prime Z370-A BIOS Version 3003 Beta

Asus Prime Z370-P BIOS Version 3003 Beta

Asus Prime Z370-P II BIOS Version 3001 Beta

Asus TUF Z370-PLUS GAMING BIOS 3002 BETA
"Add Resizable BAR support for Nvidia RTX 30 series cards to potentially deliver more performance to gamers in select titles.

Got R-BAR working on a Gigabyte Z390-M 9700K + Sapphire 5700 XT setup. Confirmed by GPU-Z!

Win for team AMD!

More good news, MSI has rolled out Beta BIOS for more of their Z370 motherboards with resizable BAR;

MSI Z370M Mortar Bios Version 7B54v1A1 Beta

MSI Z370M Gaming Pro AC Bios Version 7B44v1B2 Beta

MSI Z370I Gaming Pro Carbon AC Bios Version 7B43v1B1 Beta

MSI Z370-A Pro Bios Version 7B48v2D2 Beta

MSI Z370 Tomahawk Bios Version 7B47v1B2 Beta

MSI Z370 SLI Plus Bios Version 7B46vAB2 Beta

MSI Z370 Krait Gaming Bios Version 7B46v1C2 Beta

MSI Z370 Godlike Gaming Bios Version 7A98vAC2 Beta

MSI Z370 Gaming Pro Carbon Bios Version 7B45vAC2 Beta

MSI Z370 Gaming Pro Carbon AC Bios Version 7B45vAC2 Beta

MSI Z370 Gaming Plus Bios Version 7B61v1B1 Beta

MSI Z370 Gaming M5 Bios Version 7B58v1B2 Beta

From the benchmarks I have seen so far it looks like you need a powerful CPU (i.e. +6 cores) and GPU to realise the most gains from Re-BAR so even if Z170/Z270 had support it may not give any gains and could even hurt performance.

Zitat von davidm71 im Beitrag #48

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Got R-BAR working on a Gigabyte Z390-M 9700K + Sapphire 5700 XT setup. Confirmed by GPU-Z!

Win for team AMD!

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Got Re-Bar working on my GB X299 UD4 Pro (Bios F8a) with an RTX3090.
AC Valhalla went from 76 to 82 average FPS in WQHD.

Zitat von chinobino im Beitrag #49

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From the benchmarks I have seen so far it looks like you need a powerful CPU (i.e. +6 cores) and GPU to realise the most gains from Re-BAR so even if Z170/Z270 had support it may not give any gains and could even hurt performance.

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@mooms Yep, you sure can --> 9900K on Z170

I'm running a QQLS (9900K equivalent) on my Z170 Pro Gaming, with a RTX 3070, so resizable Bar would be great

Hello guys, when I was experimenting with a CoffeeMod BIOS on my Asrock Z170G K4, I erroneously flashed a BIOS from the similar Asrock board (B360 or something like this), strangely the PC booted and I noticed a few interesting things:
- Native Z170G K4 BIOS is laggy and has a tendency to hang from time to time in random places, being it the HW monitoring or any other tab, not even related to overclocking - while B360 one works smoothly and much faster than the original;
- There was a setting non-existing in native BIOS - not yet a ReBAR but some prerequisite to it, unfortunately don't recall the name. Given the fact it works with my mobo, it should be somehow possible to extract and embed it into the older Z170 generation of motherboards. I see that most manufacturers started to add such settings to their Z370 series (Asus, Gigabyte, some other) so apparently when Asrock manages to deploy their Z370 BIOSes, it would worth giving it a try, especially if one's mobo has dual BIOS so you can safely play with it not ruining the functioning system.

Interesting.

Was the BIOS fully functional?
Did the card boot Windows?

Now I'm wondering if I can boot my Z170 with a Z370 BIOS.

Well, BIOS itself ran awesome, all settings worked and were saved. Windows didn't boot due to the fact NVMe controllers of my board and B360 are different hence there's simply no module to support mine. I didn't try to set up a system to SATA SSD (was quite limited on time) but when exiting BIOS it gave a message that "No bootable disk is found", so I assume if I had a spare SSD to play with, it would boot just fine.
N.B. Please make backups before you try something like this as there's absolutely no warranty it would even work, I was just lucky enough that it went up afterwards :)