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

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Date:   Tue, 12 Oct 2021 19:36:56 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>
Cc:     x86@...nel.org, "Chang S. Bae" <chang.seok.bae@...el.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        kvm@...r.kernel.org
Subject: Re: [patch 16/31] x86/fpu: Replace KVMs homebrewn FPU copy to user

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