linux-kernel - Re: [PATCH v1 2/3] x86/msr: Switch between WRMSRNS and WRMSR with the alternatives mechanism

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Message-ID: <20240817144316.GAZsC3BOm7anUPdJGe@fat_crate.local>
Date: Sat, 17 Aug 2024 16:43:16 +0200
From: Borislav Petkov <bp@...en8.de>
To: Xin Li <xin@...or.com>, Andrew Cooper <andrew.cooper3@...rix.com>,
	linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, dave.hansen@...ux.intel.com,
	x86@...nel.org, hpa@...or.com, peterz@...radead.org,
	seanjc@...gle.com
Subject: Re: [PATCH v1 2/3] x86/msr: Switch between WRMSRNS and WRMSR with
 the alternatives mechanism

On Sat, Aug 17, 2024 at 05:23:07PM +0300, Borislav Petkov wrote:
> >static __always_inline void wrmsr(u32 msr, u64 val)
> >{
> >	asm volatile("1: " ALTERNATIVE_2("wrmsr", WRMSRNS, X86_FEATURE_WRMSRNS,
> >					 "call asm_xen_write_msr", X86_FEATURE_XENPV)
> >		     "2: " _ASM_EXTABLE_TYPE(1b, 2b, EX_TYPE_WRMSR)
> >		     : : "c" (msr), "a" (val), "d" ((u32)(val >> 32)),
> >		     "D" (msr), "S" (val));
> >}
> >
> >
> >As the CALL instruction is 5-byte long, and we need to pad nop for both
> >WRMSR and WRMSRNS, what about not using segment prefix at all?
> 
> The alternatives macros can handle arbitrary insn sizes - no need for any padding.

What you cannot do is have a CALL insn in only one of the alternative
insn groups because the compiler is free to clobber callee regs and
those might be live where you place the alternative and thus will have
a nice lovely corruption.

-- 
Regards/Gruss,
    Boris.

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