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Diffstat (limited to 'classification_output/02/semantic/53568181')
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diff --git a/classification_output/02/semantic/53568181 b/classification_output/02/semantic/53568181 deleted file mode 100644 index 2a2342afc..000000000 --- a/classification_output/02/semantic/53568181 +++ /dev/null @@ -1,79 +0,0 @@ -semantic: 0.943 -instruction: 0.932 -other: 0.921 -boot: 0.876 -mistranslation: 0.854 - -[BUG] x86/PAT handling severely crippled AMD-V SVM KVM performance - -Hi, I maintain an out-of-tree 3D APIs pass-through QEMU device models at -https://github.com/kjliew/qemu-3dfx -that provide 3D acceleration for legacy -32-bit Windows guests (Win98SE, WinME, Win2k and WinXP) with the focus on -playing old legacy games from 1996-2003. It currently supports the now-defunct -3Dfx propriety API called Glide and an alternative OpenGL pass-through based on -MESA implementation. - -The basic concept of both implementations create memory-mapped virtual -interfaces consist of host/guest shared memory with guest-push model instead of -a more common host-pull model for typical QEMU device model implementation. -Guest uses shared memory as FIFOs for drawing commands and data to bulk up the -operations until serialization event that flushes the FIFOs into host. This -achieves extremely good performance since virtual CPUs are fast with hardware -acceleration (Intel VT/AMD-V) and reduces the overhead of frequent VMEXITs to -service the device emulation. Both implementations work on Windows 10 with WHPX -and HAXM accelerators as well as KVM in Linux. - -On Windows 10, QEMU WHPX implementation does not sync MSR_IA32_PAT during -host/guest states sync. There is no visibility into the closed-source WHPX on -how things are managed behind the scene, but from measuring performance figures -I can conclude that it didn't handle the MSR_IA32_PAT correctly for both Intel -and AMD. Call this fair enough, if you will, it didn't flag any concerns, in -fact games such as Quake2 and Quake3 were still within playable frame rate of -40~60FPS on Win2k/XP guest. Until the same games were run on Win98/ME guest and -the frame rate blew off the roof (300~500FPS) on the same CPU and GPU. In fact, -the later seemed to be more inlined with runnng the games bare-metal with vsync -off. - -On Linux (at the time of writing kernel 5.6.7/Mesa 20.0), the difference -prevailed. Intel CPUs (and it so happened that I was on laptop with Intel GPU), -the VMX-based kvm_intel got it right while SVM-based kvm_amd did not. -To put this in simple exaggeration, an aging Core i3-4010U/HD Graphics 4400 -(Haswell GT2) exhibited an insane performance in Quake2/Quake3 timedemos that -totally crushed more recent AMD Ryzen 2500U APU/Vega 8 Graphics and AMD -FX8300/NVIDIA GT730 on desktop. Simply unbelievable! - -It turned out that there was something to do with AMD-V NPT. By loading kvm_amd -with npt=0, AMD Ryzen APU and FX8300 regained a huge performance leap. However, -AMD NPT issue with KVM was supposedly fixed in 2017 kernel commits. NPT=0 would -actually incur performance loss for VM due to intervention required by -hypervisors to maintain the shadow page tables. Finally, I was able to find the -pointer that pointed to MSR_IA32_PAT register. By updating the MSR_IA32_PAT to -0x0606xxxx0606xxxxULL, AMD CPUs now regain their rightful performance without -taking the hit of NPT=0 for Linux KVM. Taking the same solution into Windows, -both Intel and AMD CPUs no longer require Win98/ME guest to unleash the full -performance potentials and performance figures based on games measured on WHPX -were not very far behind Linux KVM. - -So I guess the problem lies in host/guest shared memory regions mapped as -uncacheable from virtual CPU perspective. As virtual CPUs now completely execute -in hardware context with x86 hardware virtualiztion extensions, the cacheability -of memory types would severely impact the performance on guests. WHPX didn't -handle it for both Intel EPT and AMD NPT, but KVM seems to do it right for Intel -EPT. I don't have the correct fix for QEMU. But what I can do for my 3D APIs -pass-through device models is to implement host-side hooks to reprogram and -restore MSR_IA32_PAT upon activation/deactivation of the 3D APIs. Perhaps there -is also a better solution of having the proper kernel drivers for virtual -interfaces to manage the memory types of host/guest shared memory in kernel -space, but to do that and the needs of Microsoft tools/DDKs, I will just forget -it. The guest stubs uses the same kernel drivers included in 3Dfx drivers for -memory mapping and the virtual interfaces remain driver-less from Windows OS -perspective. Considering the current state of halting progress for QEMU native -virgil3D to support Windows OS, I am just being pragmatic. I understand that -QEMU virgil3D will eventually bring 3D acceleration for Windows guests, but I do -not expect anything to support legacy 32-bit Windows OSes which have out-grown -their commercial usefulness. - -Regards, -KJ Liew - |