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charset=UTF-8 Content-Transfer-Encoding: 8bit On 1/15/26 08:22, Paolo Bonzini wrote: > >   Guest running with MSR_IA32_XFD = 0 >     WRMSR(MSR_IA32_XFD) >     vmexit >   Host: >     enable IRQ >     interrupt handler >       kernel_fpu_begin() -> sets TIF_NEED_FPU_LOAD >         XSAVE -> stores XINUSE[18] = 1 >         ... >       kernel_fpu_end() >     handle vmexit >       fpu_update_guest_xfd() -> XFD[18] = 1 >     reenter guest >       fpu_swap_kvm_fpstate() >         XRSTOR -> XINUSE[18] = 1 && XFD[18] = 1 -> #NM and boom > > With the patch, fpu_update_guest_xfd() sees TIF_NEED_FPU_LOAD set and > clears the bit from xinuse. Paolo, thanks for clarifying that! Abbreviated, that's just: XFD[18]=0 ... # Interrupt (that does XSAVE) XFD[18]=1 XRSTOR => #NM Is there anything preventing the kernel_fpu_begin() interrupt from happening a little later, say: XFD[18]=0 ... XFD[18]=1 # Interrupt (that does XSAVE) XRSTOR (no #NM) In that case, the XSAVE in kernel_fpu_begin() "operates as if XINUSE[i] = 0" and would set XFEATURES[18]=0; it would save the component as being in its init state. The later XRSTOR would obviously restore state 18 to its init state. Without involving SMIs, I think it lands feature 18 in its init state as well. The state is _already_ being destroyed in the existing code without anything exotic needing to happen. That's a long-winded way of saying I think I agree with the patch. It destroys the state a bit more aggressively but it doesn't do anything _new_. What would folks think about making the SDM language stronger, or at least explicitly adding the language that setting XFD[i]=1 can lead to XINUSE[i] going from 1=>0. Kinda like the language that's already in "XRSTOR and the Init and Modified Optimizations", but specific to XFD: If XFD[i] = 1 and XINUSE[i] = 1, state component i may be tracked as init; XINUSE[i] may be set to 0. That would make it consistent with the KVM behavior. It might also give the CPU folks some additional wiggle room for new behavior.