linux-kernel - Re: [PATCH v10 10/28] x86/fpu/xstate: Update the XSTATE save function to support dynamic states

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Message-ID: <87tui04urt.ffs@tglx>
Date:   Fri, 01 Oct 2021 17:41:42 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>, bp@...e.de,
        luto@...nel.org, mingo@...nel.org, x86@...nel.org
Cc:     len.brown@...el.com, lenb@...nel.org, dave.hansen@...el.com,
        thiago.macieira@...el.com, jing2.liu@...el.com,
        ravi.v.shankar@...el.com, linux-kernel@...r.kernel.org,
        chang.seok.bae@...el.com, kvm@...r.kernel.org
Subject: Re: [PATCH v10 10/28] x86/fpu/xstate: Update the XSTATE save
 function to support dynamic states

On Wed, Aug 25 2021 at 08:53, Chang S. Bae wrote:
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 74dde635df40..7c46747f6865 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9899,11 +9899,16 @@ static void kvm_save_current_fpu(struct fpu *fpu)
>  	 * KVM does not support dynamic user states yet. Assume the buffer
>  	 * always has the minimum size.
>  	 */
> -	if (test_thread_flag(TIF_NEED_FPU_LOAD))
> +	if (test_thread_flag(TIF_NEED_FPU_LOAD)) {
>  		memcpy(fpu->state, current->thread.fpu.state,
>  		       fpu_buf_cfg.min_size);

What happens with the rest of the state?

> -	else
> +	} else {
> +		struct fpu *src_fpu = &current->thread.fpu;
> +
> +		if (fpu->state_mask != src_fpu->state_mask)
> +			fpu->state_mask = src_fpu->state_mask;

What guarantees that the state size of @fpu is big enough when src_fpu
has dynamic features included?

>  		save_fpregs_to_fpstate(fpu);

Thanks,

        tglx