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

[H. Peter Anvin]

On August 17, 2024 7:43:16 AM PDT, Borislav Petkov <bp@xxxxxxxxx> 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.
>

Only if the call goes to a C function.