> On 13/10/21 08:15, Liu, Jing2 wrote: > > After KVM passthrough XFD to guest, when vmexit opening irq window and > > KVM is interrupted, kernel softirq path can call > > kernel_fpu_begin() to touch xsave state. This function does XSAVES. If > > guest XFD[18] is 1, and with guest AMX state in register, then guest > > AMX state is lost by XSAVES. > > Yes, the host value of XFD (which is zero) has to be restored after vmexit. > See how KVM already handles SPEC_CTRL. > I'm trying to understand why qemu's XFD is zero after kernel supports AMX. Do you mean in guest #NM trap KVM also alloc extra user_fpu buffer and clear qemu's XFD? But why do we need do that? I think only when qemu userspace requests an AMX permission and exec AMX instruction generating host #NM, host kernel clears qemu's XFD[18]. If guest #NM being trapped, KVM *don't* need clear host's XFD, but only allocate guest_fpu's buffer and current->thread.fpu 's buffer, and clear guest's XFD. > Passthrough of XFD is only enabled after the guest has caused an #NM > vmexit Yes, passthrough is done by two cases: one is guest #NM trapped; another is guest clearing XFD before it generates #NM (this is possible for guest), then passthrough. For the two cases, we passthrough and allocate buffer for guest_fpu, and current->thread.fpu. Thanks, Jing and the full XSAVE state has been dynamically allocated, therefore it > is always possible to do an XSAVES even from atomic context. > > Paolo
