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

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Message-ID: <4f7566b5-a743-4819-b221-88207c132f63@zytor.com>
Date: Fri, 16 Aug 2024 14:26:03 -0700
From: "H. Peter Anvin" <hpa@...or.com>
To: Andrew Cooper <andrew.cooper3@...rix.com>, Xin Li <xin@...or.com>,
        linux-kernel@...r.kernel.org
Cc: tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, peterz@...radead.org,
        seanjc@...gle.com
Subject: Re: [PATCH v1 2/3] x86/msr: Switch between WRMSRNS and WRMSR with the
 alternatives mechanism

On 8/16/24 11:40, Andrew Cooper wrote:
>>
>> 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?
> 

You can use up to 4 prefixes of any kind (which includes opcode prefixes 
before 0F) before most decoders start hurting, so we can pad it out to 5 
bytes by doing 3f 3f .. .. ..

> 
> My suggestion, not that I've had time to experiment, was to change
> paravirt to use a non-C ABI and have asm_xen_write_msr() recombine
> edx:eax into rsi.  That way the top level wrmsr() retains sensible
> codegen for native even when paravirt is active.
> 

I have attached what should be an "obvious" example... famous last words.

	-hpa

View attachment "xen.S" of type "text/plain" (1692 bytes)