linux-kernel - Re: [RFC PATCH v1 13/15] x86/msr: Use the alternatives mechanism to read MSR

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Message-ID: <edbeb41d-3c38-4778-9a7c-255edc7cd5fb@zytor.com>
Date: Thu, 17 Apr 2025 04:10:59 -0700
From: Xin Li <xin@...or.com>
To: Francesco Lavra <francescolavra.fl@...il.com>
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Subject: Re: [RFC PATCH v1 13/15] x86/msr: Use the alternatives mechanism to
 read MSR

On 4/14/2025 10:13 AM, Francesco Lavra wrote:
> This works only if this function has been called directly (e.g. via
> `call asm_xen_write_msr`), but doesn't work with alternative call types
> (like indirect calls). Not sure why one might want to use an indirect
> call to invoke asm_xen_write_msr, but this creates a hidden coupling
> between caller and callee.
> I don't have a suggestion on how to get rid of this coupling, other
> than setting ipdelta in _ASM_EXTABLE_FUNC_REWIND() to 0 and adjusting
> the _ASM_EXTABLE_TYPE entries at the call sites to consider the
> instruction that follows the function call (instead of the call
> instruction) as the faulting instruction (which seems pretty ugly, at
> least because what follows the function call could be an instruction
> that might itself fault). But you may want to make this caveat explicit
> in the comment.

Good idea, will state that in the comment.