Re: [PATCH] x86/barrier: Do not serialize MSR accesses on AMD

[Peter Zijlstra]

On Tue, Jul 04, 2023 at 09:46:31AM +0200, Borislav Petkov wrote:
> On Mon, Jul 03, 2023 at 02:54:19PM +0200, Peter Zijlstra wrote:
> > So you're saying that AMD tsc_deadline and x2apic MSRs *do* imply
> > ordering constraints unlike the Intel ones?
> 
> Yah, that's the default situation. Only those two - TSC_DEADLINE and
> x2APIC MSRs - and on *Intel* are special.

So they are normal MSRs like all other? AMD doesn't have any exceptions
for MSRs, they all the same?

> > Both instructions are 3 bytes, a 6 byte nop would be better, no?
> 
> Why? You wanna save the branch insn when sending IPIs through the
> x2APIC? Does that really matter? I doubt it...

Dunno, code density, speculation, many raisons to avoid jumps :-)

> > 	asm volatile (ALTERNATIVE("mfence; lfence;", "", X86_FEATURE_AMD));
> 
> There's no X86_FEATURE_AMD :)

I know, but that's easily fixed.