> The reason turned out to be that SVM is discarding the guest PAT setting and uses the default (PA0=PA4=WB, PA1=PA5=WT, PA2=PA6=UC-, PA3=UC). The guest might be using a different setting, and especially might want write combining but isn't getting it (instead getting slow UC or UC- accesses). [0]

That's all it is. Not a big complicated bug, the patch is seven simple lines.

[0] https://patchwork.kernel.org/patch/10027523/