On 3/7/25 08:41, Chao Gao wrote: > case |IA32_XSS[12] | Space | RFBM[12] | Drop% > -----+-------------+-------+----------+------ > 1 | 0 | None | 0 | 0.0% > 2 | 1 | None | 0 | 0.2% > 3 | 1 | 24B? | 1 | 0.2% So, 0.2% is still, what, dozens of cycles? Are you sure that it really takes the CPU dozens of cycles to skip over the feature during XSAVE? If it really turns out to be this measurable, we should probably follow up with the folks that implement XSAVE and see what's going on under the covers. On a separate note, I was bugging Thomas a bit on IRC. His memory was that the AMX-era FPU rework only expected KVM to support user features. You might want to dig through the history a bit and see if _that_ was ever properly addressed because that would change the problem you're trying to solve.
