Re: [patch 13/31] x86/fpu: Move KVMs FPU swapping to FPU core

[Paolo Bonzini]

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.

Passthrough of XFD is only enabled after the guest has caused an #NM 
vmexit and the full XSAVE state has been dynamically allocated, 
therefore it is always possible to do an XSAVES even from atomic context.

Paolo