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Date:   Wed, 20 Apr 2022 18:32:43 +0200
From:   Borislav Petkov <bp@alien8.de>
To:     Lai Jiangshan <jiangshanlai@gmail.com>,
        =?utf-8?Q?J=C3=BCrgen?= Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, x86@kernel.org,
        Lai Jiangshan <jiangshan.ljs@antgroup.com>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH V5 5/7] x86/entry: Don't call error_entry() for XENPV
Message-ID: <YmA1q3PyEEWmDtQy@zn.tnic>
References: <20220412121541.4595-1-jiangshanlai@gmail.com>
 <20220412121541.4595-6-jiangshanlai@gmail.com>
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On Tue, Apr 12, 2022 at 08:15:39PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> 
> When in XENPV, it is already in the task stack, and it can't fault for
> native_iret() nor native_load_gs_index() since XENPV uses its own pvops
> for IRET and load_gs_index().  And it doesn't need to switch the CR3.
> 
> So there is no reason to call error_entry() in XENPV.
> 
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
> ---
>  arch/x86/entry/entry_64.S | 13 +++++++++++--
>  1 file changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
> index 7b6a0f15bb20..3aca7815fe79 100644
> --- a/arch/x86/entry/entry_64.S
> +++ b/arch/x86/entry/entry_64.S
> @@ -328,8 +328,17 @@ SYM_CODE_END(ret_from_fork)
>  	PUSH_AND_CLEAR_REGS
>  	ENCODE_FRAME_POINTER
>  
> -	call	error_entry
> -	movq	%rax, %rsp			/* switch to the task stack if from userspace */
> +	/*
> +	 * Call error_entry() and switch to the task stack if from userspace.
> +	 *
> +	 * When in XENPV, it is already in the task stack, and it can't fault
> +	 * for native_iret() nor native_load_gs_index() since XENPV uses its
> +	 * own pvops for IRET and load_gs_index().  And it doesn't need to
> +	 * switch the CR3.  So it can skip invoking error_entry().
> +	 */
> +	ALTERNATIVE "call error_entry; movq %rax, %rsp", \
> +		"", X86_FEATURE_XENPV
> +
>  	ENCODE_FRAME_POINTER
>  	UNWIND_HINT_REGS
>  
> -- 

This one needs Jürgen. CCed.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette