linux-kernel - Re: [PATCH 14/19] x86/fpu: Prepare for KVM XFD

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Date:   Sat, 11 Dec 2021 00:20:22 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Yang Zhong <yang.zhong@...el.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        pbonzini@...hat.com
Cc:     seanjc@...gle.com, jun.nakajima@...el.com, kevin.tian@...el.com,
        jing2.liu@...ux.intel.com, jing2.liu@...el.com,
        yang.zhong@...el.com
Subject: Re: [PATCH 14/19] x86/fpu: Prepare for KVM XFD_ERR handling

On Tue, Dec 07 2021 at 19:03, Yang Zhong wrote:
> --- a/arch/x86/kernel/fpu/core.c
> +++ b/arch/x86/kernel/fpu/core.c
> @@ -322,6 +322,55 @@ int fpu_swap_kvm_fpstate(struct fpu_guest *guest_fpu, bool enter_guest)
>  }
>  EXPORT_SYMBOL_GPL(fpu_swap_kvm_fpstate);
>  
> +#ifdef CONFIG_X86_64
> +void fpu_save_guest_xfd_err(struct fpu_guest *guest_fpu)
> +{
> +	if (guest_fpu->xfd_err & XFD_ERR_GUEST_DISABLED)
> +		return;
> +
> +	/* A non-zero value indicates guest XFD_ERR already saved */
> +	if (guest_fpu->xfd_err)
> +		return;
> +
> +	/* Guest XFD_ERR must be saved before switching to host fpstate */
> +	WARN_ON_ONCE(!current->thread.fpu.fpstate->is_guest);

Warn and proceed?

> +	rdmsrl(MSR_IA32_XFD_ERR, guest_fpu->xfd_err);
> +
> +	/*
> +	 * Restore to the host value if guest xfd_err is non-zero.
> +	 * Except in #NM handler, all other places in the kernel
> +	 * should just see xfd_err=0. So just restore to 0.
> +	 */
> +	if (guest_fpu->xfd_err)
> +		wrmsrl(MSR_IA32_XFD_ERR, 0);
> +
> +	guest_fpu->xfd_err |= XFD_ERR_GUEST_SAVED;
> +}
> +EXPORT_SYMBOL_GPL(fpu_save_guest_xfd_err);
> +
> +void fpu_restore_guest_xfd_err(struct fpu_guest *guest_fpu)
> +{
> +	u64 xfd_err = guest_fpu->xfd_err;
> +
> +	if (xfd_err & XFD_ERR_GUEST_DISABLED)
> +		return;
> +
> +	xfd_err &= ~XFD_ERR_GUEST_SAVED;
> +
> +	/*
> +	 * No need to restore a zero value since XFD_ERR
> +	 * is always zero outside of #NM handler in the host.
> +	 */
> +	if (!xfd_err)
> +		return;
> +
> +	wrmsrl(MSR_IA32_XFD_ERR, xfd_err);
> +	guest_fpu->xfd_err = 0;
> +}

Why should any pf this be in the FPU core?

It's a pure guest issue as all of this is related to struct fpu_guest
and not struct fpu or any other core FPU state.

Thanks,

        tglx