linux-kernel - Re: [PATCH V3 6/7] x86/entry: Don't call error

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Date:   Wed, 16 Mar 2022 16:09:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH V3 6/7] x86/entry: Don't call error_entry for XENPV

On Tue, Mar 15, 2022 at 03:39:48PM +0800, Lai Jiangshan wrote:
> From: Lai Jiangshan <jiangshan.ljs@...group.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 CR3.
> So there is no reason to call error_entry() in XENPV.
> 
> Signed-off-by: Lai Jiangshan <jiangshan.ljs@...group.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 e4a07276fd1c..ec885c2107de 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 stack settled by sync_regs() */
> +	/*
> +	 * Call error_entry and switch stack settled by sync_regs().
> +	 *
> +	 * 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 CR3.  So it can skip invoking error_entry().
> +	 */
> +	ALTERNATIVE "call error_entry; movq %rax, %rsp", \
> +		"", X86_FEATURE_XENPV
> +
>  	ENCODE_FRAME_POINTER
>  	UNWIND_HINT_REGS
>  

Oooh, here we go, this is the answer to my question for patch #1, a note
in the changelog might be nice. Something like:

"This looses a Xen PV optimization, which will be restored in a later
patch. The superfluous stack switch is just that."