linux-kernel - Re: [PATCH v6 08/25] x86/fpu/xstate: WARN if normal fpstate contains kernel dynamic xfeatures

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Message-ID: <110a791e0f057b52b7395569422100ede192cbbf.camel@redhat.com>
Date:   Tue, 31 Oct 2023 19:45:39 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Yang Weijiang <weijiang.yang@...el.com>, seanjc@...gle.com,
        pbonzini@...hat.com, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Cc:     dave.hansen@...el.com, peterz@...radead.org, chao.gao@...el.com,
        rick.p.edgecombe@...el.com, john.allen@....com
Subject: Re: [PATCH v6 08/25] x86/fpu/xstate: WARN if normal fpstate
 contains kernel dynamic xfeatures

On Thu, 2023-09-14 at 02:33 -0400, Yang Weijiang wrote:
> fpu_kernel_dynamic_xfeatures now are __ONLY__ enabled by guest kernel and
> used for guest fpstate, i.e., none for normal fpstate. The bits are added
> when guest fpstate is allocated and fpstate->is_guest set to %true.
> 
> For normal fpstate, the bits should have been removed when init system FPU
> settings, WARN_ONCE() if normal fpstate contains kernel dynamic xfeatures
> before xsaves is executed.
> 
> Signed-off-by: Yang Weijiang <weijiang.yang@...el.com>
> ---
>  arch/x86/kernel/fpu/xstate.h | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/x86/kernel/fpu/xstate.h b/arch/x86/kernel/fpu/xstate.h
> index 9c6e3ca05c5c..c2b33a5db53d 100644
> --- a/arch/x86/kernel/fpu/xstate.h
> +++ b/arch/x86/kernel/fpu/xstate.h
> @@ -186,6 +186,9 @@ static inline void os_xsave(struct fpstate *fpstate)
>  	WARN_ON_FPU(!alternatives_patched);
>  	xfd_validate_state(fpstate, mask, false);
>  
> +	WARN_ON_FPU(!fpstate->is_guest &&
> +		    (mask & fpu_kernel_dynamic_xfeatures));
> +
>  	XSTATE_XSAVE(&fpstate->regs.xsave, lmask, hmask, err);
>  
>  	/* We should never fault when copying to a kernel buffer: */

I am not sure about this patch. It's true that now the kernel dynamic features
are for guest only, but in the future I can easily see a kernel dynamic feature
that will also be used in the kernel itself.

Maybe we can add a comment above this warning to say that _currently_ there are
no kernel dynamic features that are enabled for the host kernel.

Best regards,
	Maxim Levitsky