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

[Xin Li]

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