Re: [PATCH] rdmsr_on_cpu, wrmsr_on_cpu

["H. Peter Anvin" <hpa@zytor.com>]

Alexey Dobriyan wrote:
> There was OpenVZ specific bug rendering some cpufreq drivers unusable
> on SMP. In short, when cpufreq code thinks it confined itself to
> needed cpu by means of set_cpus_allowed() to execute rdmsr, some
> "virtual cpu" feature can migrate process to anywhere. This triggers
> bugons and does wrong things in general.
> 
> This got fixed by introducing rdmsr_on_cpu and wrmsr_on_cpu executing
> rdmsr and wrmsr on given physical cpu by means of
> smp_call_function_single().
> 
> Dave Jones mentioned cpufreq might be not only user of rdmsr_on_cpu()
> and wrmsr_on_cpu(), so I'm going to put them into arch/i386/lib/
> (after patch gets some more testing othen than compile and UP run)

The CPUID and MSR drivers need something like this.

HOWEVER -- and this is where things get gnarly -- the CPUID and MSR 
drivers would really like to be able to execute CPUID, WRMSR and RDMSR 
with the entire GPR register set (except the stack pointer) pre-set and 
post-captured, since it's highly likely that there are going to be 
nonstandard MSRs and CPUID levels (already witness Intel breaking the 
CPUID architecture by introducing %ecx dependencies.)

So I would like to see:

/* It probably makes sense to use the same structure on x86 and
    x86-64 */
struct x86_gpr_regs {
	u64 rax, rcx, rdx, rbx;
	u64 rsp, rbp, rsi, rdi;
	u64 r8, r9, r10, r11;
	u64 r12, r13, r14, r15;
};

void rdmsr_on_cpu(unsigned cpu,
	const struct x86_gpr_regs *in, struct x86_gpr_regs *out);
void wrmsr_on_cpu(unsigned cpu,
	const struct x86_gpr_regs *in, struct x86_gpr_regs *out);
void cpuid_on_cpu(unsigned cpu,
	const struct x86_gpr_regs *in, struct x86_gpr_regs *out);

This requires assembly to do in the nonparavirtualized case, of course. 
  I'll try to get that written up in the next day or so.

	-hpa