Re: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user

[Paolo Bonzini]

On 12/10/21 02:00, Thomas Gleixner wrote:
> -	if (boot_cpu_has(X86_FEATURE_XSAVE)) {
> -		memset(guest_xsave, 0, sizeof(struct kvm_xsave));
> -		fill_xsave((u8 *) guest_xsave->region, vcpu);
> -	} else {
> -		memcpy(guest_xsave->region,
> -			&vcpu->arch.guest_fpu->state.fxsave,
> -			sizeof(struct fxregs_state));
> -		*(u64 *)&guest_xsave->region[XSAVE_HDR_OFFSET / sizeof(u32)] =
> -			XFEATURE_MASK_FPSSE;
> -	}

After the patch, this final assignment is not done in the else case:

> +
> +	if (cpu_feature_enabled(X86_FEATURE_XSAVE)) {
> +		__copy_xstate_to_uabi_buf(mb, &kstate->xsave, pkru,
> +					  XSTATE_COPY_XSAVE);
> +	} else {
> +		memcpy(&ustate->fxsave, &kstate->fxsave, sizeof(ustate->fxsave));
> +	}
> +}

This leaves the xstate_bv set to 0 instead of XFEATURE_MASK_FPSSE. 
Resuming a VM then fails if you save on a non-XSAVE machine and restore 
it on an XSAVE machine.

The memset(guest_xsave, 0, sizeof(struct kvm_xsave)) also is not 
reproduced, you can make it unconditional for simplicity; this is not a 
fast path.

Paolo