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

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Date:   Tue, 15 Mar 2022 19:59:43 -0700
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Lai Jiangshan <jiangshanlai@...il.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        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:
> +++ 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

The second ENCODE_FRAME_POINTER is only needed for the stack-switching
(non-XENPV) case.  The second UNWIND_HINT_REGS shouldn't be needed at
all, since the first one still applies after the call.

How about something more readable like this?

.macro idtentry_body cfunc has_error_code:req

	PUSH_AND_CLEAR_REGS
	ENCODE_FRAME_POINTER

#ifdef CONFIG_XEN_PV
	/*
	 * 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 "", "jmp 1f", X86_FEATURE_XENPV
#endif

	call error_entry
	movq %rax, %rsp
	ENCODE_FRAME_POINTER
1:
	movq	%rsp, %rdi			/* pt_regs pointer into 1st argument*/

-- 
Josh