[Xen-devel] Patches for Nvidia GPU passthrough

[marcus@xxxxxxxx]

Hello all,

I've been working on enabling passthrough for newer Nvidia cards and 
drivers (GTX 980 specifically) on Xen and I'd like to document my 
findings up to now and ask for assistance. I apologize if this is not 
the correct mailing list, but I thought xen-devel is more suitable since 
we are talking about code changes in Xen anyway.
Problem with Nvidia GPUs has been (for two years now) that drivers 
detect it being running inside a VM and refuse to work (Code 43 error) 
if the card is not a Quadro or other high-end non-consumer grade GPU 
(though few other things could cause Code 43 or BSOD also). Now, since 
KVM has supported Nvidia GPU passthrough for quite a while (and I've 
personally succeeded in passing through GTX 980 using KVM on both Win 7 
and Win 8.1 VM's), I decided to port those few patches from KVM to Xen.
------------------


#### Patch #1: Spoof Xen and Hypervisor signatures:

KVM has for a while supported hiding both the "KVMKVMKVMKVM" signature 
(with "-cpu kvm=off" flag) as well as the Viridian hypervisor signature 
("-cpu hv_vendor_id="..." flag). Currently there's no such functionality 
in Xen, so I patched it in quite similar way to what Alex Willimson did 
for KVM.
Attached is a patch for Xen 4.6.1 that spoofs Xen signature 
("XenVMMXenVMM" to "ZenZenZenZen") and Viridian signature ("Microsoft 
Hv" to "Wetware Labs") when "spoof_xen=1" and "spoof_viridian=1" are 
added to VM configuration file.
The signatures are currently hard-coded, and currently there's no way to 
modify them (beyond re-compiling Xen), since HVMLoader also uses a 
hard-coded string to detect Xen and there's no API (understandably) to 
change that signature in real-time.
This works with qemu-xen-traditional without any additional changes, but 
qemu-xen requires that SeaBIOS is patched as well: 
https://github.com/WetwareLabs/seabios/commit/ec102d72fc1d7b2e6c8e9607266dc9bd4a42bce0
With spoofing on, it was possible to use official binary drivers from 
NVidia (tested version 367.27) on Arch Linux VM (without spoofing the 
driver would fail with a error message such as "The NVIDIA GPU at 
PCI:0:5:0 is not supported by the 367.27 NVIDIA driver".  However this 
was not enough on Windows VM's, as the Code 43 would occur regardless of 
spoofing.

#### Patch #2: Disable NoSnoop.

Background information and the related patch for KVM is here: 
https://patchwork.kernel.org/patch/3019371/
The fix was quite simple for Xen: Just modify the initial PCIe DEVCTL 
capabilities to disable NoSnoop, and make the capability read-only. 
Double-checking with Linux VM, I can see that NoSnoop is disabled for 
all devices (with lspci -vvv), but this would not prevent Code 43 on 
Windows VM.
##### Patch #3: Set CPUID to Core2duo

There have been few reports where forcing CPUID to Core2duo on KVM (-cpu 
core2duo) would help alleviate Code 43 problems (and also increase 
compatibility with Windows 10 VMs), so I copied all CPUID registers from 
proven-to-be-working KVM configuration using libcpuid 
(https://github.com/anrieff/libcpuid) and applied them to Xen VM. LibXL 
is also patched (attached file) to allow hexadecimal input of CPUID (to 
make it easier to convert CPUID output from libcpuid).
cpuid = [
         '0:eax=0000000a,ebx=756e6547,ecx=6c65746e,edx=49656e69',
         '1:eax=000006fb,ebx=00000800,ecx=80202201,edx=0f8bfbff',
         '2:eax=00000001,ebx=00000000,ecx=00000000,edx=002c307d',
         '3:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
         '4,0:eax=00000121,ebx=01c0003f,ecx=0000003f,edx=00000001',
         '4,1:eax=00000122,ebx=01c0003f,ecx=0000003f,edx=00000001',
         '4,2:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
         '4,3:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
         '5:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
         '6:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
       '7,0:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
        
'0x80000000:eax=80000008,ebx=756e6547,ecx=6c65746e,edx=49656e69',
        
'0x80000001:eax=000006fb,ebx=00000000,ecx=00000001,edx=20100800',
        
'0x80000002:eax=65746e49,ebx=2952286c,ecx=726f4320,edx=4d542865',
        
'0x80000003:eax=44203229,ebx=43206f75,ecx=20205550,edx=54202020',
        
'0x80000004:eax=30303737,ebx=20402020,ecx=30342e32,edx=007a4847',
        
'0x80000005:eax=01ff01ff,ebx=01ff01ff,ecx=40020140,edx=40020140',
        
'0x80000006:eax=00000000,ebx=42004200,ecx=02008140,edx=00000000',
        
'0x80000007:eax=00000000,ebx=00000000,ecx=00000000,edx=00000000',
        '0x80000008:eax=00003028,ebx=00000000,ecx=00000000,edx=00000000'
        ]

This makes the /proc/cpuinfo almost identical between KVM and Xen VMs 
running Linux. Only exceptions are flags "rep_good" (which is missing 
under Xen) and "eager_fpu" and "xsaveopt" (not seen under KVM), but as 
these are not explicitly set by CPUID but are Linux-specific flags, they 
shouldn't (?) matter on Windows VMs.
------------------

Anyway, even applying all of these patches would not alleviate Code 43. 
To be more specific, all NVidia drivers up to 364.72 would BSOD on boot 
(SYSTEM_SERVICE_EXCEPTION), and newer drivers (368.22+) would cause Code 
43. This happens on both Windows 7 Pro and 8.1 VMs. Result on qemu-xen 
and -traditional is identical. Dom0 is Qubes 3.1 (Linux 4.1.24), Xen 
4.6.1. Hardware: Intel i7-5820K, Asrock X99 WS motherboard, 32GB Corsair 
mem, EVGA GTX980.
I would love if some of you could try these patches with both newer and 
older NVidia cards. Also any suggestions, ideas and further patches 
would be greatly appreciated! :)
Thanks!

Best regards,
Marcus

Attachment: read-hex-cpuid.patch
Description: Text Data

Attachment: spoof-xen-viridian.patch
Description: Text Data

Attachment: disable-no-snoop.patch
Description: Text Data

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