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

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Message-ID: <451f9224-f0de-43c5-b178-2bb4281e5ad2@zytor.com>
Date: Sat, 17 Aug 2024 08:44:55 -0700
From: Xin Li <xin@...or.com>
To: Borislav Petkov <bp@...en8.de>, 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 8/17/2024 7:43 AM, Borislav Petkov wrote:
> 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.

Yeah, asm_xen_write_msr() is a wrapper function to xen_write_msr() which
saves/restores those regs.

My first draft patch calls xen_write_msr() directly and works fine.  But
hpa said the same thing as you said.

Thanks!
     Xin