linux-kernel - Re: [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove the definition of SWAPGS

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Message-ID: <Ymu2XC7k8Xj/vMjG@zn.tnic>
Date:   Fri, 29 Apr 2022 11:56:44 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Lai Jiangshan <jiangshanlai@...il.com>,
        Jürgen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Juergen Gross <jgross@...e.com>, x86@...nel.org,
        Lai Jiangshan <jiangshan.ljs@...group.com>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Kirill A. Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH V6 6/8] x86/entry: Convert SWAPGS to swapgs and remove
 the definition of SWAPGS

On Thu, Apr 21, 2022 at 10:10:53PM +0800, Lai Jiangshan wrote:
> diff --git a/arch/x86/entry/entry_64_compat.S b/arch/x86/entry/entry_64_compat.S
> index 4fdb007cddbd..c5aeb0819707 100644
> --- a/arch/x86/entry/entry_64_compat.S
> +++ b/arch/x86/entry/entry_64_compat.S
> @@ -50,7 +50,7 @@ SYM_CODE_START(entry_SYSENTER_compat)
>  	UNWIND_HINT_EMPTY
>  	ENDBR
>  	/* Interrupts are off on entry. */
> -	SWAPGS
> +	swapgs
>  
>  	pushq	%rax
>  	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax

I'm not sure about this: why can't XENPV do a 32-bit syscall through the vdso?

Also, looking at this, Jürgen, it looks kinda spaghetti to me:

entry_SYSENTER_compat

...

        /* XEN PV guests always use IRET path */
        ALTERNATIVE "testl %eax, %eax; jz swapgs_restore_regs_and_return_to_usermode", \
                    "jmp swapgs_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV


then at that swapgs_restore_regs_and_return_to_usermode label:

#ifdef CONFIG_XEN_PV
        ALTERNATIVE "", "jmp xenpv_restore_regs_and_return_to_usermode", X86_FEATURE_XENPV
#endif


Can we simply jump directly to xenpv_restore_regs_and_return_to_usermode
from entry_SYSENTER_compat or is that CONFIG_DEBUG_ENTRY chunk there
needed?

Thx.

-- 
Regards/Gruss,
    Boris.

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