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

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Message-ID: <90b290db-22fb-432c-aa94-73e85aee0d11@zytor.com>
Date: Sat, 17 Aug 2024 16:51:10 -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 15:59, H. Peter Anvin wrote:
> 
> RDMSR is a bit trickier. I think the best option there is to set up a 
> new trap fixup handler type that amounts to "get the address from 
> the stack, apply a specific offset, and invoke the fixup handler for 
> that address:
> 
> 
>      case EX_TYPE_UPLEVEL: {
>          /* Let reg hold the unsigned number of machine
>           * words to pop off the stack before the return
>           * address, and imm the signed offset from the
>           * return address to the desired trap point.
>           *
>           * pointer in units of machine words, and imm the
>           * signed offset from this stack word...
>           */
>          unsigned long *sp = (unsigned long *)regs->sp + reg;
>          regs->ip = *sp++ + (int16_t)imm;
>          regs->sp = (unsigned long)sp;
>          goto again;    /* Loop back to the beginning */
>      }
> 
> Again, "obviously correct" code attached.
> 

Here is an untested patch implemented the above functionality, tidied up 
and wrapped in a macro as _ASM_EXTABLE_FUNC_REWIND().

	-hpa

View attachment "0001-x86-extable-implement-EX_TYPE_FUNC_REWIND.patch" of type "text/x-patch" (6901 bytes)