linux-kernel - Re: [PATCH V4 04/50] x86/xen: Add xenpv_restore_regs_and_return_to

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Date:   Tue, 2 Nov 2021 10:49:44 +0100
From:   Borislav Petkov <bp@...en8.de>
To:     Lai Jiangshan <laijs@...ux.alibaba.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org, x86@...nel.org,
        Jan Beulich <jbeulich@...e.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>, Peter Anvin <hpa@...or.com>,
        xen-devel@...ts.xenproject.org, Andy Lutomirski <luto@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH V4 04/50] x86/xen: Add
 xenpv_restore_regs_and_return_to_usermode()

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