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From: Borislav Petkov <bp@alien8.de>
To: Lai Jiangshan <laijs@linux.alibaba.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, x86@kernel.org,
	Jan Beulich <jbeulich@suse.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Juergen Gross <jgross@suse.com>, Peter Anvin <hpa@zytor.com>,
	xen-devel@lists.xenproject.org, Andy Lutomirski <luto@kernel.org>,
	Ingo Molnar <mingo@redhat.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Stefano Stabellini <sstabellini@kernel.org>
Subject: Re: [PATCH V4 04/50] x86/xen: Add xenpv_restore_regs_and_return_to_usermode()
Date: Tue, 2 Nov 2021 10:49:44 +0100	[thread overview]
Message-ID: <YYEJuIgQukcDzy1R@zn.tnic> (raw)
In-Reply-To: <d4ae23dd-377e-8316-909b-d5bdeacc0904@linux.alibaba.com>

On Tue, Nov 02, 2021 at 05:19:46PM +0800, Lai Jiangshan wrote:
> It will add a 5-byte NOP at the beginning of the native
> swapgs_restore_regs_and_return_to_usermode.

So?

> I avoided adding unneeded code in the native code even if it is NOPs
> and avoided melting xenpv-one into the native one which will reduce
> the code readability.

How does this reduce code readability?!

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index e38a4cf795d9..bf1de54a1fca 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -567,6 +567,10 @@ __irqentry_text_end:
 
 SYM_CODE_START_LOCAL(common_interrupt_return)
 SYM_INNER_LABEL(swapgs_restore_regs_and_return_to_usermode, SYM_L_GLOBAL)
+
+	ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", \
+                X86_FEATURE_XENPV
+
 #ifdef CONFIG_DEBUG_ENTRY
 	/* Assert that pt_regs indicates user mode. */
 	testb	$3, CS(%rsp)

> I will follow your preference since a 5-byte NOP is so negligible in the slow
> path with an iret instruction.

Yes, we do already gazillion things on those entry and exit paths.

> Or other option that adds macros to wrap the ALTERNATIVE.
> RESTORE_REGS_AND_RETURN_TO_USERMODE and
> COND_RESTORE_REGS_AND_RETURN_TO_USERMODE (test %eax before jmp in native case)

No, the main goal is to keep the asm code as readable and as simple as
possible.

If macros or whatever need to be added, there better be a good reason
for them. Saving a NOP is not one of them.

Thx.

-- 
Regards/Gruss,
    Boris.

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

  reply	other threads:[~2021-11-02  9:50 UTC|newest]

Thread overview: 59+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-10-26 14:13 [PATCH V4 00/50] x86/entry/64: Convert a bunch of ASM entry code into C code Lai Jiangshan
2021-10-26 14:13 ` [PATCH V4 01/50] x86/entry: Add fence for kernel entry swapgs in paranoid_entry() Lai Jiangshan
2021-10-27 12:19   ` Borislav Petkov
2021-10-28  0:51     ` Lai Jiangshan
2021-10-26 14:13 ` [PATCH V4 02/50] x86/entry: Use the correct fence macro after swapgs in kernel CR3 Lai Jiangshan
2021-10-26 14:13 ` [PATCH V4 03/50] x86/traps: Remove stack-protector from traps.c Lai Jiangshan
2021-10-26 14:13 ` [PATCH V4 04/50] x86/xen: Add xenpv_restore_regs_and_return_to_usermode() Lai Jiangshan
2021-11-02  8:58   ` Borislav Petkov
2021-11-02  9:19     ` Lai Jiangshan
2021-11-02  9:49       ` Borislav Petkov [this message]
2021-11-02 11:22         ` H. Peter Anvin
2021-11-02 14:27           ` Borislav Petkov
2021-10-26 14:20 ` [PATCH V4 05/50] x86/entry: Use swapgs and native_iret directly in swapgs_restore_regs_and_return_to_usermode Lai Jiangshan
2021-10-26 14:20 ` [PATCH V4 06/50] compiler_types.h: Add __noinstr_section() for noinstr Lai Jiangshan
2021-10-26 14:24   ` Miguel Ojeda
2021-10-26 14:20 ` [PATCH V4 07/50] x86/entry: Introduce __entry_text for entry code written in C Lai Jiangshan
2021-10-26 14:20 ` [PATCH V4 08/50] x86/entry: Move PTI_USER_* to arch/x86/include/asm/processor-flags.h Lai Jiangshan
2021-10-26 14:20 ` [PATCH V4 09/50] x86: Remove unused kernel_to_user_p4dp() and user_to_kernel_p4dp() Lai Jiangshan
2021-10-26 14:20 ` [PATCH V4 10/50] x86: Replace PTI_PGTABLE_SWITCH_BIT with PTI_USER_PGTABLE_BIT Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 11/50] x86: Mark __native_read_cr3() & native_write_cr3() as __always_inline Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 12/50] x86/traps: Move the declaration of native_irq_return_iret into proto.h Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 13/50] x86/entry: Add arch/x86/entry/entry64.c for C entry code Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 14/50] x86/entry: Expose the address of .Lgs_change to entry64.c Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 15/50] x86/entry: Add C verion of SWITCH_TO_KERNEL_CR3 as switch_to_kernel_cr3() Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 16/50] x86/traps: Add fence_swapgs_{user,kernel}_entry() Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 17/50] x86/entry: Add C {user,kernel}_entry_swapgs_and_fence() Lai Jiangshan
2021-10-26 14:27 ` [PATCH V4 18/50] x86/traps: Move pt_regs only in fixup_bad_iret() Lai Jiangshan
2021-10-26 14:30 ` [PATCH V4 19/50] x86/entry: Switch the stack after error_entry() returns Lai Jiangshan
2021-10-26 14:30 ` [PATCH V4 20/50] x86/entry: move PUSH_AND_CLEAR_REGS out of error_entry Lai Jiangshan
2021-10-26 14:30 ` [PATCH V4 21/50] x86/entry: Move cld to the start of idtentry Lai Jiangshan
2021-10-26 14:30 ` [PATCH V4 22/50] x86/entry: Don't call error_entry for XENPV Lai Jiangshan
2021-10-26 14:30 ` [PATCH V4 23/50] x86/entry: Convert SWAPGS to swapgs in error_entry() Lai Jiangshan
2021-10-26 14:32 ` [PATCH V4 24/50] x86/entry: Implement the whole error_entry() as C code Lai Jiangshan
2021-10-26 14:32 ` [PATCH V4 25/50] x86/entry: Use idtentry macro for entry_INT80_compat Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 26/50] x86/entry: Convert SWAPGS to swapgs in entry_SYSENTER_compat() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 27/50] x86: Remove the definition of SWAPGS Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 28/50] x86/entry: Make paranoid_exit() callable Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 29/50] x86/entry: Call paranoid_exit() in asm_exc_nmi() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 30/50] x86/entry: move PUSH_AND_CLEAR_REGS out of paranoid_entry Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 31/50] x86/entry: Add the C version ist_switch_to_kernel_cr3() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 32/50] x86/entry: Skip CR3 write when the saved CR3 is kernel CR3 in RESTORE_CR3 Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 33/50] x86/entry: Add the C version ist_restore_cr3() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 34/50] x86/entry: Add the C version get_percpu_base() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 35/50] x86/entry: Add the C version ist_switch_to_kernel_gsbase() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 36/50] x86/entry: Implement the C version ist_paranoid_entry() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 37/50] x86/entry: Implement the C version ist_paranoid_exit() Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 38/50] x86/entry: Add a C macro to define the function body for IST in .entry.text Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 39/50] x86/debug, mce: Use C entry code Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 40/50] x86/idtentry.h: Move the definitions *IDTENTRY_{MCE|DEBUG}* up Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 41/50] x86/nmi: Use DEFINE_IDTENTRY_NMI for nmi Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 42/50] x86/nmi: Use C entry code Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 43/50] x86/entry: Add a C macro to define the function body for IST in .entry.text with an error code Lai Jiangshan
2021-10-26 14:34 ` [PATCH V4 44/50] x86/doublefault: Use C entry code Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 45/50] x86/sev: Add and use ist_vc_switch_off_ist() Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 46/50] x86/sev: Use C entry code Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 47/50] x86/entry: Remove ASM function paranoid_entry() and paranoid_exit() Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 48/50] x86/entry: Remove the unused ASM macros Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 49/50] x86/entry: Remove save_ret from PUSH_AND_CLEAR_REGS Lai Jiangshan
2021-10-26 14:38 ` [PATCH V4 50/50] x86/syscall/64: Move the checking for sysret to C code Lai Jiangshan

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